Digital Public Library of America (more than 37 million free-to-use images, texts, videos and sounds held within America’s libraries, archives, museums, and other cultural heritage institutions. Keyword search or explore Browse by Topic, Exhibitions, and Primary Source Sets in the top menu bar. Browse by time period within Primary Source Sets.)
Google Images Click on Tools and choose Usage Rights: Creative Commons Licenses for images you may freely use.
Library of Congress: Prints and Photographs Online Catalog: Search by the subject/contents of the photograph/print and date for best results. If you start typing a date, a series of suggested dates and/or date ranges will pop up. If you need any help, please ask a Librarian!
The New York Public Library Digital Images Collection (Be sure to check "Search only public domain materials" when you search this digital collection 881,097 prints, photographs, maps, and more.)
Public Domain Images from the National Archives (This set features images from the U.S. National Archives that are believed to be in the public domain. As NARA explains: "The vast majority of digitized historical documents and photographs in ARC are in the public domain. Therefore, no written permission is required to use them. We would appreciate your crediting the National Archives and Records Administration as the original source. For the few that remain copyrighted, please read the instructions noted in the description for the digital image.)
Wikimedia Commons (Wikimedia Commons is a free media repository of over 58 million freely usable items)
To cite an Image, choose the option that best describes where you found it:
Best Practices for Attribution with examples; from Creative Commons.
Next you will choose the type of image you are citing. For instance, a cartoon or comic strip, map or chart, photo or illustration, or work of visual art.