No matter what size I tired to change the thumbnail to, it just stay the same. The make matter worst, when I click on the LARGE thumbnail as shown on the page, it would go directly to redisplay the image and no slide show. I don't know what is going on. Please take a look.

www.cpmdemo.com/cms15/index.php?option=com_content&view=article&id=57&Itemid=64

I have the exact same problem. Using a host with PHP 5, Joomla 1.5, and a completely fresh install.  The only modules I have installed are the ones that come pre packed with Joomla of course, and Image Gallery Pro.

Thumbnails show, but then images just pop in a new window, no lightbox effect. I have heard that some module that comes with Joomla can interfere with lightbox, anyone have a link to fix this?

<jdoc:include type="head" />

Fixed it for me, not sure why I was missing that in my header.

I think it's something to do with the "Pop-up Engine". The only one working for me is the "Grey Box (based on AJS)". Don't know why but none of the other ones are working at all. I continue to get a new pop-up window to an image file when selecting the other pop-up engine. Hopefully the developer can help solve this issue.