This is a page I started compiling several months ago of websites that make available images that are either in the public domain or are open access and therefore free for us to use in our teaching and publications.
I recently learned about a similar effort by Asa Mittman on the Art History Rules! website. His site gives more info on each link because each institution has its own way of thinking about these policies. http://www.arthistoryrules.com/Art_History_Online/Copyright_Rules.html