How hard would it be to open the larger images up into a full-screen lightbox type image instead of how they open now..?

shouldn't be too hard, take a look at the theming tutorial and function reference guide, and then change the references you need to use the lightbox script.

Theming Tutorial
http://www.zenphoto.org/category/user-guide/theming-templating/

Function Guide
http://www.zenphoto.org/documentation/index.html