Artportal Image Search
Artcyclopedia: Find an Artwork. (search 50,000+ online works by title)

Image Search Engines:
Search Google Images:    Advanced image search
     Adult filter:
Search Yahoo Images:    Advanced image search
     Adult filter:
Search MSN Live Images:
     Adult filter: default is Moderate
Search Picsearch:
     Adult filter: Picsearch is intended to be family safe

Photo-Sharing Websites
(Sometimes good sources for photographs of paintings; can be very good sources for sculpture, architecture and public art)

Identifying Unknown Artworks
Artconversation - Forum where you can upload an image and ask for help identifying the artist, title or subject.

Art Viewing Tools
ActualSizer - Simple tool that displays an image of a 2-D artwork at its exact real-life size (assuming you know the size, obviously). It won't improve the quality of the image, but it makes it easier to visualize how the artwork looks in real life.
  Art Image Archives Online
Art Images for College Teaching (AICT)
Art Renewal Center
The Athenaeum
Joconde Catalogue des Collections des Musées de France (in French)
Web Gallery of Art
Visual Arts Data Service, U.K. - "Over 100,000 images free for use in education"
World Art Treasures/Jacques-Edouard Berger Foundation
Artprice Images - Images from 290,000 art auction catalogues (subscription required)

Art + General Image Archives Online
AllPosters (most images are zoomable)
Memory of the Netherlands (Koninklijke Bibliotheek)
Wikimedia Commons
WorldImages (California State University IMAGE Project)

Art Image Providers for Publication or Professional Use
Alinari Picture Imaging Photo Gallery
The Art Archive
Art Resource
Bildarchiv Foto Marburg
Bridgeman Art Library
Christie's Images
Erich Lessing Culture and Fine Arts Archives
Scala Archives

General Image Providers for Publication or Professional Use
Getty Images
iStockphoto Royalty Free Images
123RF Royalty Free Images

Clip Art
Open Clip Art Library - Free, user-contributed, "open-source" clip art.
Incredible Art Department - Clip Art Resources