Artsy Education - "Artsy’s mission is to make all the world’s art accessible to anyone with an Internet connection." Powered by the Art Genome Project.
Digital Public Library of America (DPLA) "The DPLA is a large-scale, collaborative project across government, research institutions, museums, libraries, and archives to build a digital library platform to make America’s cultural and scientific history free and publicly available online. Artstor partnered as a content hub for DPLA and has provided access to more than 26,000 high-quality images and data records from leading museums and universities."
Open F|S (Freer and Sackler Galleries images): "complete digitized collections of the Freer and Sackler Galleries and the Freer Study Collection. With more than 40,000 works being made available for high-resolution download—expanding regularly with our new acquisitions—you can explore the Smithsonian's museums of Asian art from anywhere in the world, whenever you like. Images can be used for all non-commercial purposes, from desktop wallpapers to artistic gifts for family and friends. Send us your creations and your feedback about this beta version at firstname.lastname@example.org! For permission to use high-resolution images commercially, contact Rights and Reproduction.
Wikimedia Commons - database of freely usable images and other media to which anyone can contribute