There was a great article on the internet by Kevin L Callahan, but the links dont' seem to work any more, which is a great shame. He is / was an anthropologist at the university of Minnesota, and the article was comparing different rock art round the world, I think.

-So that's a pretty pointless posting Rhiannon. But the itself name might help?

There's so many extremely knowledgeable rock art people here that I'm loathe to say anything but I personally like the idea it's all to do with trance states and images people see when they're in them. That's just my twopenn'orth though (I guess lots of theories of all sorts of things reflect the theorist more than the truth).