There is a standard ecommerce shop joomshopping [url removed, login to view] installed in joomla 2.5. Joomshopping has a lightbox feature at products flypage. (Everyone can install it in a view minutes in XAMPP)
At products list view you can click at products image an get forwarded to products flypage.
This is your work:
At products list view edit php-file and:
a) remove flypage-link at image (there should be only a text link)
b) by clicking the image the image should appear in a lightbox (like done in the flypage >tempaltes>default>product>[url removed, login to view])
As result we need the modified php-file.