Alta Vista's Image Search Engine
www.altavista.com/sites/search/simage

Berkeley's ImageFinder
sunsite.berkeley.edu/ImageFinder
Search point for numerous historical image databases, including SPIRO: Architectural Images Database (UC Berkeley Architecture Slide Library), the Fort Worth Star-Telegram, the National Library of Australia, and the Library of Congress American Memory Collections.

Finding Images on the Web
www.bu.edu/library/training/webimages.htm
A tutorial on image searching, downloading, and legal use of copyrighted images. Includes a collection of links to art, science, map, advertisement, photograph and other image collections and search engines.  Created by the Mugar Memorial Library of Boston University.

Google Image Search
http://images.google.com
Finds images by matching search terms to the captions of image files, adjacent ext and other factors.

Internet Public Library's Pathfinder for Historical Photos
www.ipl.org/ref/QUE/PF/histphotos.html
An extensive collection of links to quality digitized historical collections at libraries, museums and archives.