ARTstor contains nearly 1.4 million images in the areas of art, architecture, the humanities, and social sciences. Most (but not all!) of ARTstor's images are of archival or museum materials.
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NYPL Digital Collections Rich collection of public domain historical images and other historical material.
Library of Congress' Public Domain Archive Rich collection of public domain historical images and other historical material. Additional material available: Library of Congress Digital Collections
Specialized topics:
Guide to Finding and Using Images Online