https://www.nga.gov/open-access-images.html
https://www.nga.gov/notices/open-access-policy.html
The National Gallery of Art, located in Washington DC, welcomes all people to explore and experience art, creativity, and our shared humanity. The Gallery’s website provides online access to its entire permanent collections and exhibitions consisting of over 50,000 images of works of art available for download free of charge for any use, whether commercial or non-commercial. The Gallery has an open access policy for these images believed to be in the public domain, adding more pictures for free download as more digitalized works of art become available to the public. The Gallery’s goal is to make art available to as many people as possible to facilitate research and promote scholarship and the spread of knowledge. Users can attribute or cite the photographs acquired through the National Gallery of Art’s Public Domain Collection, though the use of the Gallery’s logo implying that the Gallery endorses your work is prohibited without previous consultation.
