The Center for Creative Photography is an art and resource center located on the University of Arizona campus. It contains archives of early twentieth-century photographers. You can search within the collection by browsing images, or by browsing collections.
The ICP collection which covers the entire history of photography, contains over 100,000 photographs. In addition to photographs, ICP's Research Center provides biographical information on photographers.
The PPOC contains public access to catalog records and digital images. Even though the collection is international in scope it is rich in materials relevant to the United States.
The Metropolitan Museum of Art provides digital images available to be licensed for study, editorial, and commercial usage. Try searching the Collections section by artist or material.
The Museum of Modern Art holds more than 25,000 works of modern and contemporary photography. To view photographic images try searching MoMA's collection by limiting the search to the Photography Department.
The Princeton University Art Museum contains holdings of over 20,000 photographs from the 1840s to the present. The photography collection includes artists such as, Alfred Stieglitz, Ansel Adams, Wynn Bullock, and Imogen Cunningham.