Beyond Wikipedia: Background & Reference Sources, from the Mina Rees Library at the Graduate Center, has many more archival sources.
NYPL Digital Collections 187,000 images in the public domain, with no restrictions on use. When you click on the search box to type in search, make sure you click on the box that says "public domain materials."
Embed Getty Images on your non-commercial website or blog
The Public Domain Review images, books, audio and film in the public domain
Pixabay "Free images and videos you can use anywhere"
Unsplash free ("do whatever you want") high-resolution photos.
US Government Photos and Images “Some of these photos are in the public domain or U.S. government works and may be used without permission or fee. However, some images may be protected by license or copyright. You should read the disclaimers on each site before using these images.”
Flickr Commons Catalog of the world’s publicly-held photo archives. The photos have “no known copyright restrictions.”
UN Multimedia Audiovisual Archive UN’s Photo Library holds approximately 800,000 photographs, with over 90,000 available for high-resolution download. Some are free to use, check restrictions.
Google’s Searchable Magazines Collection Search for page images of many magazines, not free to use. Choose and click on the magazine title you want to search and you will be able to browse all archived issues or search within all issues by keyword or name.
US Government Medical Stock Images & Footage
Note: Most US government images and graphics are in the public domain, but read the disclaimers on each site before use.
NLM Digital Collections contains Images from the History of Medicine (IHM). There are more than 70,000 historical images, from the Middle Ages to the present. You can also search for the images through LocatorPlus® and the NLM Catalog.
The NLM Visible Human Project® is a digital image library of complete, normal adult male and female anatomies.
NCI Visuals Online is from the National Institutes of Health’s National Cancer Institute.
Open-I is the Open Access Biomedical Images search engine. Access images in the National Library of Medicine's PMC (formerly PubMed Central) – an open archive of free full-text biomedical and life sciences journal literature.
The Centers for Disease Control and Prevention's (CDC) Public Health Image Library (PHIL) is a gateway to images at the CDC.
The US Census Library resources include audio, video, and infographics.