Your gallery images actually look OK in Edge to me.
I do not see any significant difference between the display of your gallery images in Chrome and Edge.

As your images should fit within your gallery's image without any dynamic scaling required, try turning of Juicebox-Pro's dynamic scaling (at least temporarily for testing purposes) to see if this makes a difference.
Set imageScaleMode="NONE" (in JuiceboxBuilder-Pro's 'Customize -> Main Image' section).

I've not checked the exact dimensions of all your images but if there is any reason for Juicebox-Pro to dynamically scale an image, you might still find that increasing your image resolution will help. You might find that a 1024 x 768 image being scaled down to fit within your 600 x 600 gallery might look better than a 610 x 400 image (for example) being scaled down to fit within the same area.

Hopefully either setting imageScaleMode="NONE" or increasing your image resolution (but not both at the same time) will help.

I tested my suggestion on Windows 10 before I posted and it works for me.
I'm sorry to hear that it does not work on Windows 7 for you (but I expect this will be the case for all Adobe AIR applications).
Is your version of AIR up-to-date (v29.0)? If not, maybe upgrading it will help. You can download the latest version of AIR from here.
Unfortunately, I do not think that there is much else you can do (other than perhaps upgrade to Windows 10 but I'm sure there are reasons why you are still using Windows 7).

If you encounter the "Config file not found." message whilst trying to view a gallery locally (from your computer rather than from your web server) in Safari 11, then please see this forum post for a full explanation and a workaround.

Unfortunately, Safari 11 (released 19 September 2017) introduced a security restriction which prevents local viewing of Juicebox galleries by default. This issue is specific to viewing galleries locally (from your computer's hard drive) and there is no such issue once the galleries have been uploaded to a web server (where they can be viewed in any modern browser).
Selecting 'Disable Local File Restrictions' from Safari 11's 'Develop' menu (full instructions in the link above) should, once again, allow you to view your galleries locally in Safari 11.

More general information on the "Config file not found." message can be found in this FAQ:
When I view my gallery I see the message 'Config file not found'. How do I fix this?

Please see this forum thread. Hopefully the links and suggestions will help.
(Deleting the ~/Library/Application SupportAdobe/AIR/ELS/JuiceboxBuilder-Pro folder has certainly helped other users experiencing a similar problem but maybe the other suggestions in the forum thread will help.)

I notice that your images are of quite a low resolution. (The first image in your gallery is only 600px x 420px.)
Try increasing the resolution of your gallery images. The default resolution for gallery images when using JuiceboxBuilder-Pro to create a gallery is 1024px x 768px (over 3 times as many pixels as your current first image).

Don't worry about your images being too large for your gallery. Your gallery uses the default value for the imageScaleMode configuration option (SCALE) so Juicebox-Pro will dynamically scale down large images so that they fit within the gallery's image area (whilst respecting their aspect ratios and without cropping).

When allowing JuiceboxBuilder-Pro to resize images for a gallery at the default resolution (1024px x 768px) and quality (80%), the resulting images should have a filesize of around 120KB so the images should take no longer to load than your current images.

Just increase your image resolution slightly and you might also like to try bumping up the quality a little (try 90%).
This should help.

No worries!
Whilst we're on the topic of the 'theme.css' file, you might like to take a look at the Theming Guide which has a few pointers towards customizing a gallery beyond what can be achieved with the available Config Options.

Your screenshot looks very strange.
The drop-down menu at the top is missing, there should be a Juicebox logo to the left of 'Start -> Images -> Customize -> Publish' and there should be no "Upgrade to Juicebox-Pro!" link in JuiceboxBuilder-Pro.
It certainly seems that your JuiceboxBuilder-Pro installation is somehow corrupt and I'd recommend a complete uninstallation/reinstallation of both JuiceboxBuilder-Lite (if you have it installed) and JuiceboxBuilder-Pro.

Please follow the procedure below.

(1) Uninstall 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.

(2) After uninstalling JuiceboxBuilder-Pro, please check that there are no files are left behind (even from a failed installation) by manually deleting the following folders (if they exist) from your hard drive:
Mac:
/Applications/JuiceboxBuilder-Pro.app/
/Users/Username/Library/Application Support/Adobe/AIR/ELS/JuiceboxBuilder-Pro
/Users/Username/Library/Preferences/JuiceboxBuilder-Pro
Windows:
C:\Program Files (x86)\JuiceboxBuilder-Pro
C:\Users\Username\AppData\Roaming\Adobe\AIR\ELS\JuiceboxBuilder-Pro
C:\Users\Username\AppData\Roaming\JuiceboxBuilder-Pro
C:\Users\Username\Documents\JuiceboxBuilder-Pro
You may need to show hidden files to find some of the above locations in Windows File Explorer.

(3) Also, please follow the procedure above (Steps #1 - #2) for JuiceboxBuilder-Lite, too (if you have it installed).
JuiceboxBuilder-Lite and JuiceboxBuilder-Pro can both be installed and run side by side on the same computer but, being that you are experiencing problems, it might be wise to completely clear your system of all JuiceboxBuilder files before trying to reinstall JuiceboxBuilder-Pro.

(4) Reinstall JuiceboxBuilder-Pro following the instructions here.

(I realise that you are using a Windows PC but I've included notes for Mac above, too, in case any Mac users are experiencing similar issues and are reading this forum thread.)

Hopefully this will help.

I'm glad that you seem to have found the root of your problem. Thank you for letting me know.
Hopefully things will run smoothly for you from now on.

Unfortunately, the link you provided results in an error 404 (file not found).
Without being able to see the page into which you are trying to embed your gallery, it's very difficult for me to troubleshoot your problem.
As I mentioned earlier, if your 'Birds' folder is a complete Juicebox gallery folder, then the embedding code I provided should work fine in any web page throughout your site without modification. If it does not work, then the problem may be due to factors such as the 'Birds' folder being an incomplete Juicebox gallery folder or permissions issues (with files or folders on your web server being too restrictive).
If you can point me towards the web page that displays the "Juicebox Error: Config file not found." message (so that I can see the problem for myself), I should hopefully be able to help you further.

Try the following.
It's an extension of the original code I posted above so you'll need to make your owm modifications to it.
The additional code detects when the AutoPlay button has been clicked (to turn on AutoPlay) whilst the last image in the gallery is currently being displayed.

var jb = new juicebox({
    autoPlayOnLoad: "TRUE",
    containerId: "juicebox-container",
    showAutoPlayButton: "TRUE"
});
jb.onInitComplete = function() {
    var count = jb.getImageCount();
    jb.onImageChange = function(e) {
        if (e.id === count && $('.jb-bb-btn-auto-play').first().hasClass('jb-status-playing')) {
            window.setTimeout(function() {
                alert('Last image in gallery has been reached and 5000ms have elapsed.');
            }, 5000);
        }
    };
    $(document).ready(function() {
        $('.jb-bb-btn-auto-play').click(function() {
            var index = jb.getImageIndex();
            if (index === count && $('.jb-bb-btn-auto-play').first().hasClass('jb-status-playing')) {
                window.setTimeout(function() {
                    alert('AutoPlay button has been clicked on last image in gallery and 5000ms have elapsed.');
                }, 5000);
            }
            
        });
    });
};

I hope that you find the code useful and can implement it into your own website.

There's no benefit (as far as the gallery is concerned) to storing the configuration options via one method over the other.
Certain configuration options (for example, those referring to the structure of the gallery) must be used in the embedding code (Embed Options) but all others can be set in either the configuration file or the embedding code (whichever is more convenient).

I'd recommend storing the configuration options in the 'config.xml' file, though (as JuiceboxBuilder does).
This means that if you edit a gallery in the future, you just need to upload the new version of the 'config.xml' file (generated by JuiceboxBuilder) rather than having to manually edit the HTML file that the gallery is embedded into.

When you set a backgroundColor, it is written to the gallery's 'config.xml' file and also included in the gallery's embedding code. As noted here, any configuration options set in the embedding code will take precedence over those set in the 'config.xml' file.

Therefore, if you change the backgroundColor in JuiceboxBuilder-Pro and upload only the modified 'config.xml' file but still have a backgroundColor included in the gallery's embedding code, then it is this backgroundColor (from the embedding code) that will be used in the gallery.

Be sure to change the backgroundColor in your gallery's embedding code and all should be well.

If, after changing the backgroundColor in your gallery's embedding code, you still do not see your chosen color, then try clearing your browser's cache before reloading your web page to be sure that your browser is using the most recent versions of your gallery files.

Also, please note that if you are expanding a gallery from a Splash Page (for example in Small Screen Mode), then the gallery will only ever be visible in its expanded mode so you will see only the expandedBackgroundColor.

I hope these notes help to clarify things.

I still have no issue displaying your gallery in my own browsers (with empty caches).
All images in your gallery load quickly and completely for me.

The gallery is generated by the 'juicebox.js' JavaScript file. Essentially, Juicebox reads the imageURL data from the 'config.xml' file and displays the images via dynamically generated <img> tags (scaling the images according to the imageScaleMode value).

I see no problem with your gallery online and the variable factors in the equation are our respective internet connections and computers/browsers. The gallery itself on your web server seems fine and works for me so it appears that the problem lies either with your internet connection or computer/browser.

Unfortunately, I cannot explain why some images do not load for you. If your browser's developer tools suggest that the images are timing out before loading completely, then this sounds like a network connection problem.
Even if the problem was a JavaScript one (which seems unlikely as you are the only user to have reported such a problem), there is little that could be done as the 'juicebox.js' JavaScript file is obfuscated and cannot be modified.

• Do you see the same problem with any (or all) of our demo galleries online here?

• Do you see the problem in all browsers (Edge, Chrome, Firefox, Internet Explorer, Opera, Safari) or just certain ones?

• Have you tried temporarily disabling any browser extensions that you may have installed in the browser(s) that demonstrate the problem?

Juicebox does not have a debug mode (at least, not in the sense that you would need).
You can set debugMode="TRUE" in your gallery's 'config.xml' file (as noted here) but this just allows you to set configuration options via a query string in the URL (please see here for details) without needing to edit the gallery's HTML or XML files.

As you have mentioned 3G/4G, please also see this FAQ (although it sounds like you may already have switched 'no-transform' on).
Why can't I view my gallery on a 3G mobile connection?