Flickr Commons - The key goal of The Commons is to share hidden treasures from the world's public photography archives.
Copyright - Under "The Commons," cultural institutions that have reasonably concluded that a photograph is free of copyright restrictions are invited to share such photograph under their new usage guideline called "no known copyright restrictions."
Library of Congress Print and Photographs Online - "The collections of the Prints & Photographs Division include photographs, fine and popular prints and drawings, posters, and architectural and engineering drawings. While international in scope, the collections are particularly rich in materials produced in, or documenting the history of, the United States and the lives, interests and achievements of the American people."
Copyright - It is your responsbility to determine copyright eligibility and/or Fair Use.