Also, there are many teacher/student friendly image sites, here are a few I have used in the past.
http://www.calisphere.universityofcalifornia.edu/
More than 150,000 digitized items — including photographs, documents, newspaper pages, political cartoons, works of art, diaries, transcribed oral histories, advertising, and other unique cultural artifacts
Copryight-Free Photo Archives - 27,000 images from NASA, NOAA, and FWS
http://gimp-savvy.com/PHOTO-ARCHIVE/
Pics4Learning - great for general topics
http://pics4learning.com/
Smithsonian Institution's Office of Imaging and Photographic Services
http://photo2.si.edu/
ARS (Agricultural Research Service) Image Gallery
http://www.ars.usda.gov/is/graphics/photos/
Great Images in NASA
http://grin.hq.nasa.gov/
NASA Multimedia Gallery (Images, video, and interactive features)
http://www.nasa.gov/multimedia/highlights/index.html
Natural Resources Conservation Service (NRCS) Photo Gallery
http://photogallery.nrcs.usda.gov/
NOAA (National Oceanic and Atmospheric Administration) Photo Library - search 20,000 NOAA images
http://www.photolib.noaa.gov/
US Fish and Wildlife Service: Pictures/Graphics
http://pictures.fws.gov/
Awesome Clipart for Kids, Teacher & Family
http://www.awesomeclipartforkids.com/
Clip Art Gallery from DiscoverySchool
http://school.discovery.com/clipart/