The National Park Service has an extensive Web site with individual entries for most national parks. Try the Apostle Islands National Lakeshore for lighthouse photos, Mojave National Preserve for desert plants and animals, and so on. The Emilio Segre Visual Archives , from the American Institute of Physics, contains thousands of photographs of physicists and astronomers. Image Archive contains hundreds of historical photographs divided by categories such as heads of state, refugees, aviation, weapons and equipment, and more. Images may be used by teachers and students but credit must be given to the archive.
All photos are in the public domain.
American Memory is the online collection of the Library of Congress, with thousands of historical photographs, maps, and documents that can be browsed by collection or searched. Teachers and students may use most of the images, but be sure to abide by any copyright notices included in the collections. The Smithsonian Institution Photographic Services Home Page provides a database of images taken by Smithsonian photographers. Categories include air and space, art, people and places, science and nature, and technology and history. The image files are available for non-commercial, personal use.