ARTstor is a digital library of more than one million images and related data in the areas of art, architecture, the humanities, and social sciences with a set of tools to view, present and manage images for research and pedagogical purposes. Register for an account on artstor.org to be able to download images.
Getty Search Gateway Through the Open Content Program The Getty provides free access to images that it holds the rights to. No permission is necessary. There are additional image collections available through The Getty, but rights should be checked.