Nothing, if you’re permitted to make copies of the source of the reproduction (e.g. you went to The Louvre and took a photograph). I would have said that you likely can’t legally print or sell an image of the Mona Lisa you find on Google Images, but Wikipedia says:
> The official position taken by the Wikimedia Foundation is that "faithful reproductions of two-dimensional public domain works of art are public domain".
Anyway, reproductions of public domain paintings are commonly produced and sold for far less than the market value of the original.