Yale University has an Open Access Policy that allows the fully free use of many of its reproductions of public domain art.
You can read the policy here.
To quote the policy:
The Open Access Policy applies to digital images of works in Yale University’s museum, archive and library collections that are believed to be in the public domain and free of other restrictions, available through Yale’s electronic interfaces. The policy does not apply to works protected by copyright, by privacy rights, or that are otherwise restricted. Visit the links toward the end of this document for more information about copyright. You can discover Yale’s open access digital images through the Discover Yale Digital Content or through individual collection websites.
The Yale Center For British Art in particular marks public domain work and provides download links for high-resolution scans of artworks.
Yale’s policy is clear, reflects best practice, and creates a major resource for Open Art History. Have a browse and see what you can find!