I just saw a link recently (on Cohost, I think?) to the Getty museum's open content program:

https://www.getty.edu/art/collection/search?open_content=true

87,000 public domain images from their collection! The Metropolitan Museum of New York has a similar collection:

https://www.metmuseum.org/art/collection/search?showOnly=openAccess

Share your favorite finds! Also if you know of more Open Access collections worth diving into, share those too.