Works of art and images of works of art are copyrighted: that means United States Copyright Law (a federal law) grants the creator of an original work the exclusive right to reproduce and distribute their work. However, the rights are limited in duration and subject to some exceptions, or limitations, which permit people the use of a copyrighted work without the copyright holder's (the creator's) permission, under certain conditions.
Copyright and Ethics of Image Use by Karen Bouchard
One often permitted use of copyrighted material is for purposes of "fair use".
The policy of "Fair Use" allows scholars, students, teachers, and others to use works that are still in copyright protection for the purpose of education and criticism. This does not mean that any use is fair as long as it is for educational purposes, but education is specifically mentioned as a favored purpose. For more info, here is a link to the MIT Libraries' guide for images and fair use:
Using Images: Copyright and Fair Use by Ellen Duranceau
Art museums' online collections websites are an excellent place to find images of the artworks they own, or comparative images for other works of art you may be studying.
Here are several large, useful collections (with image numbers):
G.R. Little Library
Elizabeth City State University