This is a selected list of links to digitized art & image archives, and is by no means comprehensive. 
Many museum sites have digitized archives of their collections online; many academic institutions link to sites as well. 

For a good beginning, explore:

ARTstor,  a digital library of approximately 550,000 images in the areas of art, architecture, the humanities, and social sciences with a set of tools to view, present, and manage images for research and pedagogical purposes,
and the Voice of the Shuttle web page, below.

Indices & Compilations
Getty Research Institute
Voice of the Shuttle: art & art history
 

Digital Collections
AcIS Imaging - Digital Access Project - New York
     Part of a digital imaging project at Columbia University
Architecture Design Images History 3D Models and more - ...
Art Images for College Teaching
Art Museum Image Consortium
Art Museum Network
     Search by region:  links to museums - many which have digitized images of collections or exhibitions.
Art on the Net
Bibliotheque Nationale de France
Christus Rex
    "CHRISTUS REX, INC. is a private, non-profit organization dedicated to the dissemination of information on works of art preserved in churches, cathedrals and monasteries all over the world."  This page (at first glance) looks like a commercial site: please follow bulleted links for quality image archives.
Digital Archive of American Architecture
      Boston University, featuring over 600 images of American architecture from the 17th -20th century.
Digital Archive of European Architecture
Exploratorium
     Official website of the San Francisco science museum.
Fine Arts Museum of San Francisco
Gallery of Graffiti Art
Getty Center
Image Database links
Images of Medieval Art and Architecture
IMLS Digital Collections Registry (Institute if Museum & Library Services)
MOAC: Museums and the Online Archive of California
Museum of Fine Arts, Boston
Museum of Modern Art: Digital Works and Online Projects
Museum Web Network
National Gallery of Art
Perseus Project -  Tufts University
Tate Gallery

Individual Artists' Sites
Search individual names of artists to locate sites with digitized images.