Thank you very much for providing the link to your gallery's web page.
It is so much easier to troubleshoot a problem when I can see it live on a web server.
I see a few potential problems which I'll run through.
First of all, if I try to view the image that you are trying to use for your gallery's Back Button directly in a browser (https://galleries.ziegelmeyerphotograph … icebox.png), the browser presents a warning page (rather than the image itself) relating to an error with the SSL certificate for the domain that the image is on.
This is the reason for the broken image icon (where the Back Button should be) in the gallery.
If you click thorough the warning (in Firefox, it's 'Advanced -> Accept the Risk and Continue') and reload your gallery's web page, you'll see the image as expected.
The solution to this would be to sort out the certificate problem with the domain (or host the image elsewhere).
The next problem is that your backButtonText encloses the <img> tag in an <a> tag without a 'href' attribute.
The <a> tag overrides Juicebox's own handling of the BackButton (causing the backButtonURL to not be used) but the fact there the <a> tag does not have a 'href' attribute means that clicking the Back Button image will not redirect the user to anywhere.
Use just an <img> tag for your backButtonText (not something like <a><img ...></a>) and the backButtonURL will be used.
The next problem I see is that your logo is so large that, on my laptop screen, the Back Button image overlaps and obscures the thumbnails in the gallery.
I'd recommend making your logo a little smaller so that it does not obscure other gallery controls.
You can do this in the BackButtonText's <img> tag (without actually having to resize your image) with something like the following:
<img src="image.png" width="200" height="100">
(If you just use a 'width' attribute without a 'height' attribute (for example), then the image will be displayed at the specified width and the height will be automatically calculated respecting the image's aspect ratio.)
Finally, it's not a functional problem but you have a space before and after the URL in the <img> tags's 'src' attribute.
This is not going to cause any problems but the spaces should not really be there.
I hope these notes help you to resolve your problem.
Please let me know how you get on and if I can be of any further assistance.
Thank you.