Thanks for sharing your experience silver-net-surfer.

I understand what you mean but I'm not sure I understand how and where to locate the z-index for each module. I have the Firebug plugin installed on my firefox browser. Could it help?
But in the case the Zen Lightbox module is under an other one, will it only prevent it to work properly, and not show the popup image? Because currently, no image at all is showed in the product description. See:
http://www.offroadrescuenetwork.com/...roducts_id=181 This product has 3 working images
Also, even if I uninstall the Zen Lightbox plugin with running the uninstall SQL lines, the image are still not showing. That another reason why I think the issue is on the template. Probably juste a line that I modified that I don't remember... 