The Met has made every image they have that's not under copyright accessible under CC0.

https://www.metmuseum.org/about-the-met/policies-and-documen...

Museums in general have recognized that digital images don't replace the museum experience—and they'd rather provide good images of the art than have people rip off poor-quality ones. Not every institution, and artist copyright remains a huge barrier, particularly for contemporary art, but a LOT of artwork photography is out there in the public domain.