I made this list of free graphics sites for a friend a few days ago, and just as I was about to delete it from my desktop I realized that someone else might find it useful. So here you go.
A few of these sites may be a mix of public domain and paid license, so be sure to check for details about legal usage. It’s a mix of historic and contemporary, drawings, paintings, and photos.
http://search.getty.edu/gateway/search?q=&cat=highlight&f=%22Open+Content+Images%22&rows=10&srt=a&dir=s&pg=1 (Getty Search Gateway – very new, thousands of images)
http://publicdomainreview.org/ (Look under “Collections.” This site also has articles and film clips.)
http://www.loc.gov/index.html (Library of Congress)