There are more than 30 million freely usable media (image and/or audio files) in this database. All are licensed with Creative Commons or released into the public domain. Search by keyword, subject, or format.
The search bar is located in the upper right corner. When you click on an individual item, the copyright status should be provided in the lower right corner of your screen.
Flickr allows you to limit your search by copyright license. Select "no known copyright restrictions" or "U.S. government works" for items in the public domain. If you aren't finding enough, expand to "all creative commons".
This site allows users to browse photos or images produced by the U.S. government. These works are automatically placed within the public domain. Browse photos by subject ("Science and Technology", "Health and Nutrition", "Environment, Energy, and Agriculture", etc.) or government agency ("White House Photo Gallery", "U.S. Fish and Wildlife Service", "National Institutes of Health", etc.)
The Getty Museum houses European paintings, drawings, sculpture, decorative arts, manuscripts, architectural sketches, and photography dated from ancient eras to the present. Through it's Open Content Program, the Getty makes available, without charge, all digital images to which the Getty holds the rights or that are in the public domain to be used for any purpose. Search for digital images in the Getty Search Gateway - all open content images are identified with a "Download" link.