http://www.photolib.noaa.gov/about.html - This site would be great for science and social studies courses. NOAA (National Oceanic and Atmospheric Administration). All images are public domain and cannot be copywritten. They encourage educational use of the photos. Credit has to be given to the NOAA and the photographers.

http://karenswhimsy.com/public-domain-images/ - This site provides images specific to the needs of educators. All are from material printed prior to 1923 and they are public domain.

http://www.stockvault.net/ - This site doesnt have a very high collection of images, but has logos, clip arts and more, but the images they do have are very high in quality. For personal use only.