Look at these articles for discussions on museums and copyright.
This is a selective list of galleries and libraries through which you may search for images. For a more complete listing of U.S. and International museums, take a look at the Art Museum page of the Finding Images guide.
Google images can now be searched by reuse rights! Simply type in your search query, click "Search tools", and filter by license.
Many of the pictures in the Library of Congress Prints & Photos collection are public domain, however, it is your duty to determine if the photo you wish to use has any restrictions. Review this link for more information. Below is an RSS feed highlighting the Library of Congress Prints & Photos collection.