It might work to add some contrast to it so there is a large separation between the darkness of the outer cave, and the innder cave. Right now it seems just the color changes, not necessarily the lighting. And some shading on the cave could give it more of a rounded look as right now it looks somewhat flat.
http://i44.tinypic.com/ogevpc.gif
If you want to keep your original kind of style, it could stay flat and unshaded, but just with more contrasted colors.
Oh, I see what you mean, thank you very much.