Image Links

http://www.photodisc.com
This is a great site intended for designers to find quality stock photography and illustrations.  It works well for educators, too. My favorite feature allows the user to search using specific terms and concepts.  Searching for a “brush cut out" will produce images of brushes on white backgrounds.  Click on a thumbnail to view the larger versions of a picture. 

http://edupics.com/
This site is most applicable for elementary school and is really wonderful. It offers hundreds of high quality coloring pages.  Take advantage of it now while it is new and free, I suspect that it will go the way of the pay sites in time.

http://lcweb2.loc.gov/pp/pphome.html
This amazing offering by the Library of congress contains over 1 million digitized images from their collection. It's an incredible historical resource for primary sources. You can search in specific libraries or search in all of them simultaneously.  However, some of the libraries do not contain images for downloading.

http://www.freefoto.com
This site has 63633 free photographs with decent resolution organized by different themes and categories. It includes a search function.