The New York Metropolitan Museum of Art has just released 375,000 images of works from its collections. There are no restrictions on what you can use them for. They've been added to the US public domain.
You'll find works by Vermeer, Degas, Rembrandt, as well as historical objects and relics.
You can also take a look at over a million images from the British Library.
The New York Public Library also has over 180,000 images.