Here are more excellent sources for images in the public domain and creative commons-licensed content:
Flickr Advanced Search - Under Any License, select “all creative commons”
Google Advanced Image Search - Use the “Usage Rights” field to limit by license type.
Cleveland Museum of Art - Open access image search for CC0-licensed images
J. Paul Getty Search Gateway - open content (public domain or no known rights) images available for re-use marked with "Download"
Images of Empowerment - All photographs available under a CC BY NC 4.0
Library of Congress: Prints & Photographs Online Catalog - Photographs, prints, drawings, posters, and architectural drawings, and more.
NGA Images - Public domain artworks from the collections of the National Gallery of Art.
NYPL Digital Gallery - Illuminated manuscripts, historical maps, vintage posters, rare prints, photographs, and more, from the New York Public Library.
Open F|S - Over 40,000 Asian art images from the Smithsonian's Freer Gallery of Art and Arthur M. Sackler Gallery available for non-commercial use.
Unsplash - High quality images freely licensed for any use.
Wellcome Images - All images are made available under Creative Commons licenses.
Wikipedia Public Domain Images - List of public domain image sources on the web.