Yeah, i suppose it could be a puzzle, as for making it so stealing images is harder, it would be kinda server intensive, so i wouldn't go with that myself.
The puzzle idea is great, so is the stealer prevention. If they want to print screen then they can, but this could stop people from leeching from a site as easly for sure. A question, would this change the performance of the page at all? Just thinking that would make alot of html if you used it on alot of images.