The Metropolitan Museum of Art
The Met's Open Access program releases images of artworks it believes to be in the public domain under Creative Commons Zero (CC0). The Met says you can use those images for any purpose, including commercial use, without asking.
Not every object is included. Works still in copyright, and some works with other restrictions, are left out. The Met's collection API flags each object with an isPublicDomain field, which is what we check.
On each object page on metmuseum.org, open access works carry a "Public Domain" label near the image, with a download link.
No works here yet.