Museums with CC0 collections
Images that are explicitly marked as CC0 from these museums can be used without further research. Not all of their images are CC0; you must confirm the presence of a CC0 license on the specific image you want to use.

Rijksmuseum (Open the “Object Data” section and check the “Copyright” entry under the “Acquisition and right” section to confirm CC0.)

Met Museum (CC0 items have an “OA Public Domain” icon under the picture, which leads to the Met's Open Access Initiative page that clarifies a CC0 license.)

National Museum Sweden (CC-PD items have the CC-PD mark in the lower left of the item’s detail view.)

Minneapolis Institute of Art (Public domain items are listed as such under “Rights.”)

The Walters Art Museum (Public domain items are listed as "CC Creative Commons License" which links to a CC0 rights page.)

Art Institute of Chicago (CC0 items say CC0 in the lower left of the painting in the art detail page.)

Cleveland Museum of Art (CC0 items have the CC0 logo near the download button.)