Hello Chrischall,
I ran into the same problem when I installed it aswell, not sure if this will help you since your x-cart version is different and you may have a different dynamic image module version.
Here goes, the below code is missing in your html source code right above the "<div id="photos">"
Code:
<link rel="stylesheet" href="popups/....css" type="text/css" /><script src="popups/....js" type="text/javascript"></script>
The "..." depends on which popup option you're using. I believe you're using Thumbnailviewer as your popup. (Check your module configuration). Which the thumbnailviewer.js file located in popups/thumbnailviewer is maybe corrupted??. What I did to solve the problem is copied his thumbnailvewier.js on Telma Firma site or can be locate on this site:
http://www.dynamicdrive.com/dynamicindex4/thumbnail.htm.
Attach is the thumnailvewier.js we're using, which we've copied directly from Telma Firma's site and replace the one located in popups/thumbnailveiwer.
I hope this helps.
Thanks,
(*oops posted the wrong thumnailviewer site - fixed *)