There are probably some more Flickr accounts and museum/library collections we could add to this list. See also below on searching Google Images for images without copyright constraints.

Free Stock photo sources

Unsplash

Pexels

Pixabay

RawPixel (not all free, but much free; photos, illustrations, vectors, PSD…)

Stock photos of women of color in tech, now free to use.

nappy.co: "Beautiful photos of Black and Brown people, for free."

Broadly/Vice gender diverse stock photos

Age-positive photography from the UK Centre for Aging Better (press release)

WNYC free stock photography (modest, but fun to see news nerds in the photos)

http://www.publicdomainpictures.net/

http://www.freeimages.co.uk/ (no search index, just browse by category, some weird googlish search)

http://www.freeimages.com/ minimal non-ad results

https://morguefile.com (also searches several $$ stock services)

Lost and Taken - Texture stock photos

Smithsonian Institution Open Access

New York Public Library Digital Collections (search box shows an “only public domain” option)

Library of Congress “free to use” collections

Flickr search with ‘no known copyright restrictions’ turned on