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Images (even copyrighted ones) can generally be used for educational use under the "Fair Use" provision of U.S. Copyright Law. Scenarios like: writing about art, creating exhibits, creating art, and teaching about art fall into these categories. The College Art Association has published a document regarding "Best Practices" to help guide students and faculty regarding the appropriate use of visual materials.
High-quality authoritative images of art. Covers many time periods and subjects including architecture, painting, sculpture, photography, decorative arts, design, anthropology, and ethnographic and women's studies, as well as many other forms of visual culture.
Public Domain images are no longer covered by copyright and can be used freely without obtaining permission. Listed below are a sample of some of the places online where you can obtain Public Domain images.