Add lightbox to product list in Ubercart
Maybe you should create the product list with Views instead. Not only can you easily add lightbox like you write yourself, but you can make a lot of customizations should the need arise.

If you don't want to use Views, it's most likely the taxonomy module that generates the product list, that's how it's done on many UC sites. You could install the devel_themer and inspect the product list page with it. It should allow you to see what theme functions is being used to create the page. This should also help you figure out how you can add the customizations your want via template files or theme overrides.