This wiki was begun by a graduate student doing an internship at Wellesley College. It has lists of resources you can use to find a variety of images. The copyright restrictions may vary but they are usually noted on the image websites. Here is an example of one of the entries on the wiki:
"Mark Harden's "Artchive" contains images of many essential works of art covered in any survey course. You won't find many obscure artists or rare works here, but if you need a high-quality reproduction of one of the essentials this is an excellent resource. The site is easy to navigate by the alphabetical listing of all the artists and periods/styles on the left of the screen. Each entry has some text and a list of images towards the bottom."