The Waters Art Museum has added 17,900 new images to the Wikimedia Commons. Google’s indexed them all, and so they can now be searched via Google Images thus…

site: “Walters Art Museum”

Filtering by 2 megapixel size shows that the Walters Art Museum now has 14,400 decently-sized images accessible via Google Images, and they’re without watermarks. The test download image I tried was found to be at 300dpi. The licence is Creative Commons Attribution ShareAlike — somewhat questionable, since the works that are in the public domain might more usefully be CC Attribution.