By default, Juicebox has a gradient caption background. It is dark at the bottom and transparent at the top.
If you were to add a white image to your gallery, you should see this gradient clearly.

The problem is that Adobe AIR's integrated browser which JuiceboxBuilder uses for the live preview is unable to render all elements accurately (for example thumbnail frames, drop-shadows and gradients) so you see just a solid caption background.
This is noted in the Known Issues section of the JuiceboxBuilder User Guide.
Your own browser (Internet Explorer in your screenshot) is displaying the gallery accurately.

You can change the caption background via the captionBackColor and captionBackTopColor configuration options.
Please see the Caption Options section of the Config Options page for details.
If you want to remove the gradient, just make sure that captionBackColor and captionBackTopColor are both set to the same value.

Please note that captionBackColor and captionBackTopColor are Juicebox-Pro options only and are not supported by Juicebox-Lite.

Thank you for the link to your website.

I see that your website is essentially a single page site (as far as a visitor to your site is concerned).
The browser never leaves the main page and a selected gallery page is loaded into a frame on demand.

If possible, I would recommend embedding a gallery directly into a web page rather than using a frame (or an iframe) as there are certain drawbacks to using frames (such as the inability to expand a gallery to fill the browser window). Please see 2) Using an iframe for details.

I'm not sure that there's going to be a quick and easy way to convert all your current galleries to Juicebox ones.
There are no similarities between the structure of your current galleries and Juicebox galleries and there is no direct migration path.

There is an added complication that your current gallery pages also include information about the selected iris variety so you wouldn't be able to just swap your existing gallery pages for Juicebox ones (unless you intend to incorporate all the text into your Juicebox galleries as image captions).

I'm not quite sure what I would do if I were in your position. Whatever you do, I think you'll probably have a good deal of work ahead.
I might try to incorporate the text accompanying each gallery into the gallery itself as an image caption.
I might then use JavaScript code to load the selected gallery into a container on your main page, replacing the frame.

This brings us to the structure of your galleries and site.
If you are creating all your Galleries with JuiceboxBuilder-Pro, then it would be easier to just leave all the galleries in their folders and upload the all the folders to your web server. There would be no need to change any paths in the 'config.xml' files and editing or updating galleries would be a very easy process.
You could then load the galleries into your main page using the baseUrl switching method. Please see 'Switching between Multiple Galleries using JavaScript' in the Embedding Multiple Galleries support section for details.

If you really want to keep all your images in a single folder, then editing all the 'config.xml' files does not need to be a time consuming task.
With a text editor such as Notepad++, you can edit multiple files at once so you could open all your 'config.xml' files in the editor and a search and replace action could be applied to them all. As all your images (and thumbnails) are all in the same folder, the path to the images (the prefix you would have to add to all your imageURL and thumbUrl entries) would be the same so editing all your files should be quite quick and easy with just a couple of search and replace actions.
However, I'm not sure there would be any advantage to leaving all the images in your 'irises' folder and there would certainly be benefits to using the default gallery structure (no need to edit any files and easy to maintain individual galleries).

If you are looking for an automated solution to creating and listing multiple galleries, then you might be interested in another one of our products - Showkase.

Showkase is a PHP web application which allows you to create a complete portfolio web site (integrating multiple galleries) online.
Showkase has full support for Juicebox-Pro (and SimpleViewer-Pro) galleries and the galleries can be created within the application itself in a web browser interface (or created with JuiceboxBuilder and imported).

You can create Gallery Index pages and have as many galleries listed on each Gallery Index page as you wish.
Each gallery is represented by a thumbnail image with the gallery title displayed below and the gallery is opened when the user clicks on the image.
Demo sites created with Showkase can be found here and a sample Gallery Index page can be found here.

Showkase can also create non-gallery pages (About, Basic and Contact pages) where you can add information about yourself (or any other content you like).
All of this is done automatically within the Showkase interface without the need for any manual coding at all.

Showkase can be purchased as Showkase-Standard (which comes with Juicebox-Lite and SimpleViewer-Standard, the free versions) or Showkase-Pro (which comes with Juicebox-Pro and SimpleViewer-Pro).
The only difference between Showkase-Standard and Showkase-Pro is the bundled viewers.

However, Showkase does not have any built-in ability search functionality (like your current web site has) and you might find that listing a large number of galleries on each gallery index page is not an ideal solution for you.

I'm really just thinking out loud and letting you know of a few alternatives that you might like to consider.

Ordinarily, if you want to have 1,000 different galleries on your web site, then you will have 1,000 gallery folders to deal with.

You could have all your images stored in a single folder on your server (as you currently do) but you would need to manually adjust the paths to the images in the gallery configuration files as JuiceboxBuilder would have no knowledge of the fact that you want to store your images in a custom location when you create your galleries.
Even using a global search and replace action in a capable text editor, this would take some time to do for 1,000 galleries.

It would be easier to keep each gallery in its own folder. It helps to keeps things organized on your server and you can just swap out an entire gallery folder if you want to update a gallery in the future.

Perhaps the most efficient way to do what you are looking to achieve would be to use a Flickr account as your source of images and tag your images accordingly.
Each gallery could then be configured to display only certain tagged images (using the flickrUserName and flickrTags options).

You should be able to tag images in bulk on Flickr so it might not take too long to set up.

You can then have all your galleries share a single 'jbcore' folder (rather than having one per gallery) to minimize the upload.
This has the added advantage that when a new version of Juicebox is released, you need to update just a single 'jbcore' folder on your server (rather than 1,000). Please see the Using an External jbcore Folder support section for details.

You can simplify things further by having all your galleries share a single dummy configuration file.
All galleries need to have a 'config.xml' file (even if its content is not used) but you could upload just one for all your galleries to share (all galleries would point towards the same 'config.xml' file via the configUrl option) and you could set all your configuration options in the embedding code (in your HTML pages). Please see the Setting Config Options section.

Now, with your images hosted and tagged on Flickr and with a single 'jbcore' folder and 'config.xml' file on your server, all you would need for each galley is unique embedding code in its HTML page.
For example, the embedding code for one gallery might look something like this:

<!--START JUICEBOX EMBED-->
<script src="path_to_shared_jbcore_folder/jbcore/juicebox.js"></script>
<script>
    new juicebox({
        containerId: "juicebox-container",
        galleryWidth: "100%",
        galleryHeight: "600",
        backgroundColor: "#222222",
        configUrl: "path_to_shared_configuration_file/config.xml",
        useFlickr: "TRUE",
        flickrUserName: "your_flickr_username",
        flickrTags: "iris_number_seven"
    });
</script>
<div id="juicebox-container"></div>
<!--END JUICEBOX EMBED-->

Alternatively, you could maybe have just one gallery and have its content change dynamically.
You could perhaps have a drop-down menu of all your iris varieties. When a user selects one, you could run a JavaScript function to load or reload the gallery with the chosen subject (setting the flickrTags option dynamically).
Take a look at this Lite Flickr demo gallery to see something similar in action. You can view the source of the web page in a browser and copy or modify the code to suit your own needs.
(The demo has a text box to allow any Flickr tags to be entered but, as your scenario has a known collection of tags, you could present the user with a drop-down list of existing tags to choose from.)

If 1,000 items in a drop down list is too many, then you could maybe have a few subheadings and split the idea into a few galleries (each with 100 or so drop-down entries).

When an iris variety is selected, the JavaScript function could do more that just reload a gallery if you wanted it to.
It could change or display other content on the web page. You could maybe have a synopsis of the chosen iris variety below the gallery which could change when the gallery is reloaded. Knowledge of JavaScript would be required but it could prove to be quite a powerful solution.

I hope this gives you some food for thought.

Maybe this FAQ will help in sorting your Flickr images:
How do I change the order of images from Flickr?

This is certainly a problem related directly to Google+ (rather than Juicebox) and you might have better luck posting in a Google+ support forum.
However, it looks like Google are slowly decommissioning Picasa Web functionality.
As a test, I just created a new gallery under Google+ and although it appears in my Picasa RSS feed (so it appears to be public), its Google Album Id does not seems to be working in the Picasa Web Album API (and, consequently, the album cannot be displayed by WP-Juicbox).
Only Google really know what functionality they provide and there seems to be little definitive information about this subject on the web. (If you do a web search for 'make google photos album public' you'll find a few forum threads relating to this subject but it's as much information as I've found.)
The best information available is perhaps experimental evidence gained from your own findings and my own tests.
It looks like it is no longer possible to change the accessibility (private vs public) of a Picasa Web Album (although I cannot be certain about this, hence my suggestion to post your query in a Google+ forum) or to use the Picasa Web Album API in conjunction with a new (post-Picasa) Google+ Album Id.
Unfortunately, there is nothing that we can do to change this and, as far as I am aware, there is no workaround (other than to use a different platform to store and organise your images such as Flickr).

You're welcome!

You can rename your 'config.xml' file if you like and then use the configUrl configuration option in your gallery's embedding code to point towards it.
Please see the Embed Options here.
Sample embedding code might look something like this:

<!--START JUICEBOX EMBED-->
<script src="jbcore/juicebox.js"></script>
<script>
    new juicebox({
        configUrl: "jbconfig.xml",
        containerId: "juicebox-container",
        galleryWidth: "100%",
        galleryHeight: "100%",
        backgroundColor: "#222222"
    });
</script>
<div id="juicebox-container"></div>
<!--END JUICEBOX EMBED-->

If I go to https://picasaweb.google.com/data/feed/ … 8088651929 I am presented with an RSS feed of all your albums so 117363164378088651929 is certainly a valid Google User Id.
However, the Google Album Id 6315232953461261745 (from the URL in your last post) does not appear to be a valid Google Album Id for that account. That Google Album Id does not appear anywhere within the RSS feed data.

I do not know which album you are trying to display but if you use a Google Album Id from the links in the RSS feed page, the corresponding gallery should display fine.

You're welcome.
I'm glad your getting on well with Juicebox and your web site.

In case you're interested, I find the Mozilla Developer Network is a good reference for all things HTML, CSS and JavaScript.
You might also like to check out W3Schools. It's maybe not as comprehensive as the Moz Dev Network but it's often easier to navigate through their web pages (less information to trawl through) and find something quickly.
I also use the jQuery JavaScript Library a lot. It can simplify the code required for complex JavaScript tasks.

This is due to captionBackTopColor having different default values for Small Screen Mode and Large Screen Mode.
Please see my posts in this forum thread for more information.
The solution is to ensure that a captionBackTopColor value is explicitly set in your gallery's configuration options (and this value will be used in both Small and Large Screen Modes).
Just set captionBackTopColor="rgba(0,0,0,0)" (in JuiceboxBuilder-Pro's 'Customize -> Color' section) and all should be well.

Initially, none of your buttons have the class 'selected'.
All you need to do is give a button the class 'selected' either directly via CSS, e.g.:

<li><a href="javascript:;" id="button-1" class="selected">Catwalk Vibrant Colours</a></li>

... or via (jQuery) JavaScript:

$('#slideNav li a:eq(0)').addClass('selected');

... where 'eq(0)' represents the first button, 'eq(1)' represents the second, etc.

Try setting your gallery's background color in the JavaScript embedding code instead of just in the 'config.xml' files.
Try changing:

function loadGallery(a, b, c, y, z) {
    new juicebox({
        containerId: "juicebox-container",
        baseUrl: y,
        galleryHeight: z,
        maxThumbColumns: a,
        maxThumbRows: b,
        thumbsPosition: c
    });
    tracker = true;
}

... to:

function loadGallery(a, b, c, y, z) {
    new juicebox({
        backgroundColor: "rgba(255,255,255,1)",
        containerId: "juicebox-container",
        baseUrl: y,
        galleryHeight: z,
        maxThumbColumns: a,
        maxThumbRows: b,
        thumbsPosition: c
    });
    tracker = true;
}

This should hopefully help.

You're welcome.
Thanks for testing the fix.
I'll make sure that it's included in the next version of WP-Juicebox.