I am glad that you have been able to find the controls you are looking for.

For anyone else reading this thread and wanting to change the size of the thumbnails, go to the 'Images' tab in JuiceboxBuilder-Pro and click the 'Change Sizes...' button in the 'Image Size' control panel. You can now enter new values for the thumbnail width, height (and quality). When you save your gallery on the 'Publish' tab, thumbnails will be generated at the specified dimensions and the thumbWidth and thumbHeight configuration options will automatically be set accordingly in the gallery's 'config.xml' file.

Thank you for reporting this problem.
I have logged a bug report with the developers and it should hopefully be fixed in the next version.
In the meantime, the workaround (as you have discovered) would be to set showThumbsOnLoad="TRUE".

I notice that your gallery uses Juicebox-Pro v1.4.0.
Please try upgrading to the latest version (v1.4.1). Instructions for upgrading Juicebox-Pro can be found here.
This should fix the momentary display of the Back Button on the Small Screen Mode thumbnail page.

In Small Screen Mode, the Back Button is displayed only on the main image pages (and not on the thumbnail page).
Two possible workarounds would be to either:
(1) Force the gallery to use Large Screen Mode on all devices and in all browsers by setting screenMode="LARGE" (in JuiceboxBuilder-Pro's 'Customize-> General' section).
... or:
(2) Prevent the thumbnail page from being displayed in Small Screen Mode by setting showSmallThumbsOnLoad="FALSE" and showSmallThumbsButton="FALSE" (in JuiceboxBuilder-Pro's 'Customize-> Thumbnail' section).

Your 3G connection (not just a slow connection) is certainly the reason for the blank screen.
Please see this web page for details on how to set the Cache-Control header to 'no-transform' on IIS. (Your web host may be able to help you with this.)
Once this has been done, your Juicebox galleries should be visible by everyone (without the need to worry about your own 3G connection and the 'Request Desktop Site' functionality which would work only for yourself and not for others).

I should have mentioned earlier the imagePreloading configuration option. This determines how images are preloaded (and also controls whether or not images are kept in memory after being viewed).
Possible values for imagePreloading are PAGE, NEXT, ALL and NONE.
For short descriptions of each of these possible values, please see the the imagePreloading entry in the Main Image Options section of the Config Options page.
This will work for both Flickr and local images.

By default, the caption area is positioned on the image overlay (captionPosition="OVERLAY") and the default behavior for the overlay is to appear when the user hovers over the gallery and disappear when the user hovers out of the gallery.
If you want the overlay to be displayed at all times, you can set showImageOverlay="ALWAYS" (in JuiceboxBuilder-Pro's 'Customize -> Main Image' section). You could also choose to set showInfoButton="TRUE" ('Customize -> Button Bar'). This overrides the default behavior of the overlay and allows the user to toggle the overlay on and off via the Info Button on the Button Bar.
Alternatively, you could position the captions somewhere other than on the overlay (for example BELOW_IMAGE, BOTTOM or BELOW_THUMBS) where they will always be displayed (except for during image transitions when they fade out and back in again).

This layout would not be possible. You can certainly position the thumb dots below the main image (using thumbDots="TRUE" and thumbsPosition="BOTTOM") but it is not possible to position the caption area to the right of the main image. The caption area can be positioned using the captionPosition configuration option, whose possible values are OVERLAY, OVERLAY_IMAGE, BELOW_IMAGE, BOTTOM, BELOW_THUMBS and NONE.
For short descriptions of each of these possible values, please see the the captionPosition entry in the Config Options page.

If you would like to make any suggestions for future versions of Juicebox, please post then in the Feature Requests thread.
It keeps them all together and ensures that they are not overlooked. Thank you.

I have downloaded all the files in your gallery folder (including the .DS_Store files which are not generated by JuiceboxBuilder-Pro) and have recreated your gallery's structure on my own hard drive.
I have no problem opening it in JuiceboxBuilder-Pro (v1.4.1) on my own computer so there does not seem to be a problem with the gallery itself. This was likely to be the case but at least now we can rule out gallery errors as a possible cause of the problem. The problem certainly seems to be isolated to your own system somehow.

To summarize the above (and for anyone else experiencing a similar problem), here is a comprehensive list of things to check/try:

(1) Make sure that you are logged in with a User Account which has administrative rights. If necessary, create a new Administrator User Account and install Adobe AIR and JuiceboxBuilder-Pro from within this new account. (Windows).

(2) Make sure that you are using the latest version of Adobe AIR (v14.0) which can be downloaded from here.

(3) Install Adobe AIR not just by double-clicking the 'AdobeAirInstaller.exe' installation file but by right-clicking it and selecting 'Run as administrator...'. (Windows)

(4) Uninstall any existing version of JuiceboxBuilder-Pro:
Mac: Delete the 'JuiceboxBuilder-Pro' file from the Applications folder and empty your Trash.
Windows: Use the uninstaller in the 'Control Panel -> Programs -> Uninstall a program...' list.

(5) Make sure that no files are left behind from any existing version (or failed install) by manually deleting the following folders from your hard drive:
Mac: /Users/your_username/Library/Application Support/Adobe/AIR/ELS/JuiceboxBuilder-Pro
Windows: C:\Program Files (x86)\JuiceboxBuilder-Pro
Windows: C:\Users\Username\AppData\Roaming\Adobe\AIR\ELS\JuiceboxBuilder-Pro
Windows: C:\Users\Username\AppData\Roaming\JuiceboxBuilder-Pro
Windows: C:\Users\Username\Documents\JuiceboxBuilder-Pro
You may need to Show hidden files to find some of the above locations in Windows Explorer.

(6) Search your hard drive for the term 'JuiceboxBuilder-Pro' and delete any and all entries found (to be absolutely sure that nothing remains from a previous or failed install).

(7) Ensure that the 'JuiceboxBuilder-Pro.air' installation file is on your Desktop or in a non-restricted folder on your hard drive (and not on an external or network drive).

(8) Disconnect/unmount all drives other than the main hard drive (including time machine backups). (Mac)

(9) Ensure that any security software that you may have installed is not interfering with the installation process.

(10) Install JuiceboxBuilder-Pro following the 'Installation' instructions in the JuiceboxBuilder-Pro User Guide.

Try installing JuiceboxBuilder-Lite (from the Juicebox-Lite zip package available from this web page).
You can install and run both JuiceboxBuilder-Lite and JuiceboxBuilder-Pro side by side on the same computer.

If you can open the gallery with JuiceboxBuilder-Lite but not with JuiceboxBuilder-Pro, then this indicates that there may be something wrong with your JuiceboxBuilder-Pro installation, in which case, I would, once again, try closing JuiceboxBuidler-Pro, deleting the ELS folder (from Step #5 above) and re-opening the application.
This has been known to solve problems with the buttons on the 'Start' tab not functioning.

If you cannot open the gallery in either JuiceboxBuilder-Lite or JuiceboxBuilder-Pro, then there may be something on your system preventing the gallery folder from being opened and blocking the functionality of the 'Open Gallery...' button. Please check to see if you have any security software installed which might be interfering with the JuiceboxBuilder applications.

Juicebox-Pro provides built-in support for displaying local images or images from a Flickr account only (and not other sources such as Google+).
It is certainly possible to pull in images from a custom source but you would need to use the method outlined in this FAQ:
Can Juicebox handle a custom data source, for example RSS or Instagram?

You can use the Picasa Web Albums API to fetch images from a Google+ Album.

Try the following (I have already tested it and found it to be working):

(1) In your gallery's embedding code, set the configUrl to be configUrl: 'config.php'. For example:

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

(2) Create a new file named 'config.php' in your gallery folder containing the following code:

<?php

header('Content-type: application/xml');

$dom_doc = new DOMDocument('1.0', 'UTF-8');
$dom_doc->formatOutput = true;

$settings_tag = $dom_doc->createElement('juiceboxgallery');

$picasa_user_id = ''; // Enter your Google+ User ID here
$picasa_album_name = ''; // Enter your Google+ Album ID here

$attachments = array();
$picasa_feed = 'http://picasaweb.google.com/data/feed/api/user/' . $picasa_user_id . '/albumid/' . $picasa_album_name . '?kind=photo&imgmax=1600';
$entries = simplexml_load_file($picasa_feed);
if ($entries) {
    foreach ($entries->entry as $entry) {
        $attachments[] = $entry;
    }
}

if ($attachments) {
    foreach ($attachments as $attachment) {
        $media_group = $attachment->children('http://search.yahoo.com/mrss/')->group;
        $image_url = $media_group->content->attributes()->{'url'};
        $image_element = $dom_doc->createElement('image');
        $image_element->setAttribute('imageURL', $image_url);
        $image_element->setAttribute('thumbURL', $media_group->thumbnail[1]->attributes()->{'url'});
        $image_element->setAttribute('linkURL', $image_url);
        $image_element->setAttribute('linkTarget', '_blank');
        $title_element = $dom_doc->createElement('title');
        $title_text = $dom_doc->createCDATASection($attachment->title);
        $title_element->appendChild($title_text);
        $image_element->appendChild($title_element);
        $caption_element = $dom_doc->createElement('caption');
        $caption_text = $dom_doc->createCDATASection($attachment->summary);
        $caption_element->appendChild($caption_text);
        $image_element->appendChild($caption_element);
        $settings_tag->appendChild($image_element);
    }
}

$dom_doc->appendChild($settings_tag);

echo $dom_doc->saveXML();

?>

You will need to enter your Google+ User ID and Album ID in the 'config.php' file where indicated.
Make sure your Google+ Album is made public ('Sharing options -> Visible to -> Public -> Save').
When you view a Google+ Album, the URL will look something like:
https://plus.google.com/photos/012345678901234567890/albums/9876543210987654321
The User ID is the number which comes after '/photos/' and the Album ID is the number which comes after '/albums/'.

Alternatively, if you wanted to pull in images from a Google Drive folder (rather than a Google+ Album), then you would need to find a way to fetch a list of files from the Google Drive folder and build the gallery's XML file in a manner similar to the example above.

I hope this points you in the right direction.

Sorry, my mistake.
Entering <a><img src="jbcore/classic/img/Logo-Dark.png" /></a> as a value for the backButtonText will work only in JuiceboxBuilder-Pro.

If you enter the value directly into a gallery's XML file, you will need to escape all XML entities as noted in this FAQ:
How do I add HTML formatting to the Gallery Title or Back Button?

Therefore, the code you would enter when editing your gallery's XML file directly would be:

backButtonText="<a><img src="jbcore/classic/img/Logo-Dark.png" /></a>"

Sorry for the inconvenience but this should work fine.

@quimic

If you like, you can post suggestions for future versions of Juicebox in the Feature Requests thread.
I do not know the likelihood of feature requests being implemented but posting them in this thread keeps them all together and ensures that they are not overlooked by the developers.

Both formats I posted above work in Safari 5.1.7 on PC and Mobile Safari on iOS 6.1.6.
If you find that:

linkURL="http://www.example.com/gallery/#2"

... does not work in one (or more) of your browsers, then it would be safer to use:

linkURL="http://www.example.com/gallery/index.html#2"

... which should work in all browsers.

If your gallery is in a folder which is synced with a cloud storage service or on a restricted file system or network drive, try moving the gallery folder to a different location on your hard drive (such as your desktop) before trying to open it.