Oooooh I know this one! I've used several of the following in collages.
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The Met's Open Access allows netizens access to over 492,000 images of public domain artwork from its collections, INCLUDING basic information about the works if you're interested in their history!
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The Smithsonian has something similar, with even more content to it - not only does their Open Access include images for collage, but several different FREE dev tools, 3d models, and open research datasets in a variety of disciplines.
- Same with
the Library of Congress. They have rights-free images, maps, and movies sorted by a variety of topics, "from Classic Children’s Books and Disability Awareness to Dragons, Football, Jazz, and Shoes."
- I haven't used this one yet, but the
Public Domain Image Archive looks promising, and there's even an "Infinite Random View" where you can just scroll through random images until you find something you like.
I hope this helps in some way! I love collages (and the site you linked to - SUCH a cool concept for a webpage, love their work!) and want to help other people love them too.