Using & Citing Images (a resource guide)
Guide to Finding Public Domain Works Online (The Public Domain Review)
Over two million high-quality images in the arts, architecture, humanities, and social sciences. NOTE: Artstor is now in JSTOR
Over two million digital images of art, history, and culture from global museums, galleries, private collections and contemporary artists all copyright cleared for educational use.
Including the Historic American Buildings Survey/Historic American Engineering Record/Historic American Landscapes Survey, federal programs that have documented thousands of historic sites with measured drawings, photographs, and written histories since 1933