The Rijksmuseum offers high-res digital downloads free for non-commercial use. https://www.rijksmuseum.nl/en/
Currier Museum of Art: A quick resource guide for fair use images here: http://currierartlibrary.
NGA Images (National Gallery of Art in Washington DC) https://images.nga.gov/en/
LACMA unrestricted images collection:
The Walters Art Museum
The British Museum Free Non-Commercial Use Images:
Art Institute of Chicago Ryerson and Burnham Libraries. Another great resource for digital images http://digital-libraries.saic.
Getty Open Content Program: High resolution images of works from the Getty Museum and Getty Research Institute that are in the public domain and may be used freely. http://blogs.getty.edu/iris/
Brown University. The list is annotated and includes notes when images are freely available: https://sites.google.com/a/
University of Colorado, Boulder: http://cuart.colorado.edu/
UCLA : You might also find some use in the Creative Commons, Open Content, & Public Domain images tab of my Image Resources research guide at: http://guides.library.ucla.
Yale Digital Commons has 250,000 images "without license." http://discover.odai.yale.edu/
Artsy (public domain images): http://artsy.net/post/
CAA (College Art Association) is also keeping a list at http://www.collegeart.org/standards-and-guidelines/intellectual-property/image
Flickr Commons: almost everything is downloadable. http://www.flickr.com/commons/
Wikimedia Commons tries to aggregate a number of institutions' free content. http://commons.wikimedia.org/
One thing I always have hesitations about, in these lists we call "free" or "open" image sources, is that when I read the fine print on the museum sites there are often qualifications. Even Creative Commons licenses depend on the uploader in fact having copyright ownership of what s/he is uploading. There are Picasso paintings on Flickr with the photographer (not Picasso) giving an open access license.