Legal issue is only one aspect of the bigger problem. It is also very discouraging when you see stolen photo or just little modified one and that post has like 30 votes and 100$ in money. People trying hard to post something original earning some cents for their work on the other side.
There should be some kind of code that will prevent you from posting exactly something that is already available on web (image or text). But how will you code whales and different groups to actually reward good content and not only friends. That is even bigger issue to me.