Search [July 24, 2012, 6:47 a.m.]
Here are some places where you can find photos in the public domain or under open licenses:
The web has so much content available, but what are you allowed to use? Search for photos that are in the public domain or shared under open licenses. When doing your search, keep in mind what you will be using the image for, as you may want to revise your search to reflect the permissions you need. For example, if you are planning to sell your resulting work, you will need to find a photo that allows commercial reuse.
Flickr
The Commons on Flickr hosts images with "no known copyright restrictions" from various institutions, including the Library of Congress, National Archives UK, the Smithsonian, and more. You can search The Commons directly at http://www.flickr.com/commons/.
Flickr is also one of the largest sources of CC licensed photos on the web: http://www.flickr.com/creativecommons/. You can also search for CC licensed images via Flickr's advanced search: http://www.flickr.com/search/advanced/?.
Wikimedia Commons
Wikimedia Commons is the media repository that serves Wikipedia and other wiki projects. It has photos and other kinds of media in the public domain and under various open licenses: http://commons.wikimedia.org/wiki/Main_Page
Creative Commons Search Portal
Other repositories of public domain and CC licensed photos exist on the web. CC's search portal provides convenient access to some of these at http://search.creativecommons.org/.
Having trouble finding the image you're looking for? Try using different key words.
Are there other great websites where you searched for open photos? What are they?