Photos for Class Search for copyright free images AND it cites it for you.
Creative Commons Great search engine to search Flickr, Google Images, and more for images you can use.
NYPL Digital Images - free (somewhat historical) images that have been digitized from the New York Public Library
Photo Pin Easy search system, lots of interesting images.
FlickRiver Searches Flickr for images. Easy browsing system.
Wikipedia List of Public Domain Sources Links to a lot of GREAT collections of images you are free to use. Lots of subject specific collections.
American Memory Project Place to find historical images to use.