Remember to check museum websites for digital images. The quality of these images will be better than most you will find on the open web. And remember to always cite your images, no matter where you find them!
Metropolitan Museum Initiative
Provides Free Access to 400,000 Digital Images
The Metropolitan Museum’s initiative—called Open Access for Scholarly Content (OASC)—provides access to images of art in its collection that the Museum believes to be in the public domain and free of other known restrictions; these images are now available for scholarly use in any media. Works that are covered by the new policy are identified on the Museum’s website (http://www.metmuseum.org/collections) with the acronym OASC.