Why isn't Lightbox Working?

I’m using the theme Sutra for this site, but I’ve modified the colors.

I have the plugin Simple Lightbox installed.

The plugin worked with EVERY theme I’ve tried….but this one. Can someone tell me why? It darkens the screen, but the photo doesn’t show up.

Here’s a sample of what’s going on:

http://epplerart.com/epplerart.com/test/collection-of-works/new-works/