Also, there are many teacher/student friendly image sites, here are a few I have used in the past.
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

Pics4Learning - great for general topics

Smithsonian Institution's Office of Imaging and Photographic Services

ARS (Agricultural Research Service) Image Gallery

Great Images in NASA

NASA Multimedia Gallery (Images, video, and interactive features)

Natural Resources Conservation Service (NRCS) Photo Gallery

NOAA (National Oceanic and Atmospheric Administration) Photo Library - search 20,000 NOAA images

US Fish and Wildlife Service: Pictures/Graphics

Awesome Clipart for Kids, Teacher & Family

Clip Art Gallery from DiscoverySchool