I would not mind, after the game is over, maybe have a list of where the artwork is from.
As far as the game, I'm liking what you're doing with your "find the difference" games.
This is a good game.
Good Game :D
Difficulty needs to be upped a bit, though. After finding one, 3 others in the frame can be found, usually because it's like "Oh hey, I just found a dragon. Maybe there's other differences around the dragons. Yep, here's three more!"