Wikimedia Commons: A searchable collection of art and photography used on Wikipedia, some of which is public domain.
Flickr Commons: A searchable of art and photography which is public domain or creative commons.
New York City Public Library Digital Collections: An impressive collection of digitized artwork and photography, sorted by topic, genre, etc…
Internet Archive: is what it sounds like, an archive of TONS of stuff on the internet. This does include not only individual images but entire books, and lots of other things.
Old Book Illustrations: Search a collection of old book illustrations, or look through categories of them.
Awesome Art: A website with art sorted by the artist and to some degree time and place. They have additional art which can be purchased, but quite a bit that you can download freely.
Getty Museum Collection: A collection of public domain art and photography of sculptures and more. I’ve only just started looking through this one, but some of it is of exceptionally high quality, and should be great for print work.
Rijksmuseum: A museum in Amsterdam. Their online collection is very high quality.
The British Museum’s Online Collection: a collection of photographs of historical artifacts and art.