The MET: “You’ll want to filter by “Open Access” and make sure the image you’re downloading says “Public Domain” under it on the detail page.” Some items available under creative commons license.
The Art Institute of Chicago: You’ll want to click on filter by “Public Domain” and make sure the image you’re downloading says “Public Domain” under it on the detail page.
Paris Musées – the fourteen museums of the City of Paris (not sure of rights, although some images are in the public domain and described so. Search for ‘grenouille’ returned a page of results including political art (frogs with human heads, frogs with rabbit ears, etc.), searching for ‘frog’: no results.)
Rijksmuseum – rights are listed for each work
Useum – anything with a “download” button is free to use, even for commercial use. Includes Rijksmuseum, Los Angeles County Museum of Art, National Gallery of Art in Washington D.C., Metropolitan Museum, National Gallery of Denmark, J. Paul Getty Museum, Indianapolis Museum of Art,
British Museum: A total of 1.9 million British Museum images released under a Creative Commons 4.0 license, free to download, adapt and use for non-commercial purposes—so long as the museum is credited.