You're welcome.
I'm glad you've got it working. Thank you for letting me know.

Please check your email. I have sent you a message with an email address where you can forward me your login details.
Thank you.

As an update to this topic, the code I posted back in 2015 worked at the time but it seems to be unreliable now (the arrow keys work for only one image transition before the gallery needs a click for keyboard controls to continue functioning).
I find that the following code seems to work well with the current version of Juicebox (v1.5.0):

jb.onInitComplete = function() {
    $('.juicebox-gallery').first().focus();
};

@ryanrowlett
If you'd like to incorporate this focus code into the Lightroom plugin (so that each 'index.html' file generated by the plugin includes this code), then you can edit the plugin's template 'index.html' file as follows.

(1) Open the plugin's template 'index.html' file in a plain text editor. (Change the directory name from 'juicebox_pro.lrwebengine' to 'juicebox_lite.lrwebengine' is you are using the Lite version of the plugin.)
Mac - Users/username/Library/Application Support/Adobe/Lightroom/Web Galleries/juicebox_pro.lrwebengine/index.html
Window - C:\Users\username\AppData\Roaming\Adobe\Lightroom\Web Galleries\juicebox_pro.lrwebengine\index.html

(2) Scroll down to the start of the 'new juicebox' embedding code section.

(3) Replace:

new juicebox({
    backgroundColor: "<%= get_rgba(model.extra.backgroundColorColor, model.extra.backgroundColorOpacity) %>",
    containerId: "juicebox-container",
    galleryHeight: "<%= model.extra.galleryHeight %>",
    galleryWidth: "<%= model.extra.galleryWidth %>"
});

... with:

var jb = new juicebox({
    backgroundColor: "<%= get_rgba(model.extra.backgroundColorColor, model.extra.backgroundColorOpacity) %>",
    containerId: "juicebox-container",
    galleryHeight: "<%= model.extra.galleryHeight %>",
    galleryWidth: "<%= model.extra.galleryWidth %>"
});
jb.onInitComplete = function() {
    $('.juicebox-gallery').first().focus();
};

As you are also a Showkase user, I should point out that this will work only if the gallery's 'index.html' page is used to display the gallery (on a page of its own).
If the gallery is subsequently imported into Showkase, then this modification will be lost. The gallery's HTML embedding page is not used when importing a gallery (only the images and configuration options in the gallery's 'config.xml' file are taken into consideration).
If you'd like the focus code to be used for Juicebox gallery pages within Showkase, then just add the following code to your theme's 'custom.js' file.

$(document).ready(function() {
    if (jb) {
        jb.onInitComplete = function() {
            $('.juicebox-gallery').first().focus();
        };
    }
});

For example, if you are using the Kosel, them, then the 'custom'js' file can be found in this location: showkase/_themes/kosel/js/custom.js
If you are using a different theme, just replace 'kosel' in the path above with the name of the theme that you are using.

It would be great if you could please post your suggestions for future versions in the Feature Requests forum thread.
This keeps all the ideas together and ensures that they are not overlooked by the developers.
Many thanks!

You can skip the thumbnail page in Small Screen Mode and go straight to the  main images by setting showSmallThumbsOnLoad="FALSE" (in JuiceboxBuilder-Pro's 'Customize -> Thumbnails' section). You can also hide the Thumbnail Button on the Button Bar if you like (so users cannot access the thumbnail page at all) by setting showSmallThumbsButton="FALSE".

There are similar configuration options available for Large Screen Mode to initially hide or show the thumbnails (showThumbsOnLoad) and to choose whether or not to display the Thumbnail Button (showThumbsButton).

Additionally, you can choose which image to initially display via firstImageIndex ('Customize -> General').

The image you are describing is the Splash Page image.
More information about the Splash Page can be found in the Screen Modes section of the Gallery Tour.

By default, the Slash Page uses the first image in the gallery and scales the image to fill (rather than fit within) the Splash Page (which uses the gallery dimensions). If the image does not have the same aspect ratio as the gallery/Splash Page, then cropping will occur.

There is no way to change the scaling behavior for the Splash Page image but you can choose an image to be used for the Splash Page via the splashImageUrl configuration option.
The splashImageUrl can be an absolute path or a relative path (relative to the web page containing the gallery's embedding code).
The splashImageUrl does not need to be a gallery image so you could perhaps prepare an image that represents your gallery and looks better at the gallery/Splash Page dimensions.

For reference, all the Splash Page configuration options can be found here.

No, there's no limit to the number of galleries you can have on a single web page.

That's great to hear! I'm glad you've been able to resolve your problem. Thank you for posting back to let me know.

For anyone who wants to embed multiple galleries on a single web page, here are a few tips.
(1) Load the 'juicebox.js' file just once per web page (rather than once per gallery).
(2) Embed each gallery into a div container with a unique containerId.
(3) In each gallery's embedding code, use a configUrl (to point towards a unique configuration file) or a baseUrl (to point towards a unique gallery folder).

Short descriptions of the configuration options in bold above can be found in the Embed Options section of the Config Options page.

@racerx2oo3 & @studio tissot mayenfisch

Thanks for offering to share your solutions and feedback!
I hope you get on well with your implementations.

Try temporarily reverting to a default WordPress theme (such as Twenty Seventeen) just in case this makes a difference.

I am happy to help further but I would really need access to both your web server and WordPress site (to upload some test files are try things out) in order to determine the cause of your problem.
If you are agreeable to this, then please let me know and I will send you an email address where you can forward me your FTP and WordPress login details.
Thanks you.

Juicebox-Pro uses one set of configuration options for both Large Screen Mode and Small Screen Mode so, unfortunately, it is not possible to configure each screen mode differently. Additionally, certain configuration options are Large Screen Mode only (such as captionPosition and imageTransitionType) and these cannot easily be changed at all for Small Screen Mode.
There's more information about Screen Modes and how Juicebox adapts to different devices and screen sizes here.

What you might like to try, however, is force the gallery to always be displayed in Large Screen Mode (no matter what device or browser is being used to view the gallery) by setting screenMode="LARGE" (in the 'General' configuration options section).

If you want to see the Small Screen Mode version of your gallery in your desktop browser, temporarily set screenMode="SMALL".
Alternatively, if you use Firefox, you could perhaps try using 'Tools -> Web Developer -> Responsive Design Mode'.
Another suggestion would be to use an extension such as User Agent Overrider which, as the name suggests, overrides the user agent string for your browser (to identify your browser as something that it's not). You could select 'iOS / Safari 10', for example, and your gallery should be displayed in Small Screen Mode (as if on an iOS device). This is not an emulator but it should be suitable for examining the Small Screen Mode version of your gallery in your browser's developer tools.
There should be similar options and extensions for other browsers.

I hope this points you in the right direction.

You're welcome! I'm glad you've got it working.
Thank you for letting me know.

If you are not already using the latest version of WP-Juicebox (v1.5.0.1), then please try upgrading.
You can download the latest version from the plugin's support page here.
After upgrading, please clear your browser's cache (to ensure that no older cached files are being used) and try again.

If you are already using the latest version, then this error message is most often associated with the use of a third-party editor (rather than the default CKEditor which comes bundled with WordPress). If you are using a third-party editor, try temporarily disabling it to see if this helps.

The next most likely cause might be whitespace being introduced by a core WordPress file.
If you have manually edited any core WordPress files (such as 'wp-config.php'), make sure that you have not introduced any whitespace (e.g. line feeds or carriage returns at the end of the file outside the PHP section) that would be output when the file is parsed.

I hope these suggestions help.
However, if you continue to experience difficulties, please let me know and I'll try to help further.
Thank you.