Some of these images are "free" -- for others there is a fee. "Royalty-free" means that you don't have to pay royalty fees for use, but you may still have to pay a fee for the image. Visit these sites with caution as the pop-up ads can be invasive.
(UM only) ARTstor is a database of nearly 1 million images brought together to serve the scholarly and instructional needs of art historians, as well as students and faculty in the humanities and related social sciences.
Online access to over 500,000 photos with detailed captions from the AP news wire. Includes all photos from US and abroad added in the previous 365 days (800 per day) plus many selected older photos from 1844 to the present. Updated continuously.
OCLC's Catalog of Art Museum Images Online — is a growing online collection documenting works of art from around the world, representing the collections of prominent museums. CAMIO highlights the creative output of cultures around the world.
Historical Collections for the Library of Congress Digital Library. American Memory is a gateway to rich primary source materials relating to the history and culture of the United States. The site offers more than 7 million digital items from over 100 collections.