Digital Public Library of America
http://dp.la
A portal to collections (including photographs, video and images) from hundreds of libraries and archives across the country. DPLA is working with heritage institutions to streamline Rights & Permissions for researchers and users.
Flickr Commons
https://www.flickr.com/commons
When searching, use the “Search the Commons” lower on the page. A collective effort by archives worldwide to “to share hidden treasures from the world’s public photography archives.” The site encourages users to add tags and information to help catalog image. The FAQ has a wonderful explanation of “No Known Copyright Restrictions.”
Library of Congress
www.loc.gov
The Library of Congress website is a tremendous resource for historical images and documents from American history. Select the Prints and Photographs Collection or Film/Video collection for your image search.
New York Public Library Digital Collection
http://digitalcollections.nypl.org/
Click in the search box and a “Search only public domain materials” checkbox appears. Click that option to limit your searches to materials you can use without copyright restriction. Along with a huge variety of photographs, available images include 40,000 stereoscopes, maps, and sheet music.
United States National Archives
http://www.archives.gov/research/alic/reference/photography.html
In addition to government records, the National Archives and Records Administration (NARA) collections offer photographs. The link above goes to a page that includes links to many regional and local archives and libraries.