The Mona Lisa is not copyrighted. It is in the public domain. You can copy it all you want.
Except that… well, a picture of the Mona Lisa is copyrighted. So you can’t take a picture out of an art book and put it up on your web page. How would they know? They would analyze your copy and look for faults. If your faults are the same as the photographer’s faults, he can sue you. But first he’d have to admit, I think, that he was a lousy photographer.
If you took a picture of someone painting a copy of the Mona Lisa, you could be in trouble: his painting of the Mona Lisa is copyrighted, even if it’s an exact copy. You’d have to black out his picture. But then, you still have a picture of him. Can he sue your for violating his personal copyright of his own face? I don’t think so.
What if you took a picture of someone’s picture of a painting of someone else’s pictures? Sure, maybe he’s got a copyright on his picture of the painting, but what right does he have to take that picture of the painting of other paintings? Does he have permission from the artists who did the paintings in the painting? I’ll bet he doesn’t. So if he sues you, maybe you can find the descendents of the artists and sue him.