Firstly, great plugin. Thanks!
Secondly, an apology. I'm a first time theme author, so this is definately my fault. The plugn works fine on TwentyEleven, so the problem is with my amateur theme.
Fancy box attempts to load the image and overlay, but instead only manages a small white square with an outer shadow.
The loading animation does play, then *proof* small white square and nothing else.
I'm hoping that this is just something silly that I've messed up and is obvious to you. Any help would be geatly appriecated.