AP Images: Over 3.5 million news photographs from the 1840s through today. Also includes maps, timelines, logos and graphs. AP Interactives: A collection of multimedia content from the archives of the Associated Press. Interactives are fully-licensed for educational use.
The ARTstor database includes the Magnum Photo Collection, which contains 80,000 photographs from the Magnum photojournalism agency. (You must establish an Artstor account from an computer on the American University network before gaining off-campus access.)