The J. Paul Getty Trust has released a searchable database of 4,600 open content images (as reported by the Open Knowledge Foundation). You can now search specifically for open content images, which do not need to be cleared for copyright. Remember, though, a citation is still good academic form!
A Frog Sitting on Coins and Holding a Sphere: Allegory of Avarice, by Jaques de Gheyn II, c.1609, source: J. Paul Getty Museum.