Most major museums are making images of their art works available to the public. Many, but not all, are available in open access or public domain.
Art Institute of Chicago
The Collection - Type CC0 in the search box to view the over 50,000 images in the museum collections that are in the public domain or to which the museum has waived copyright. Most images also include descriptive information, a bibliography for the artwork, and other resources.
J. Paul Getty Museum
Getty Search Gateway
Louvre (Paris)
Search the Collection
Metropolitan Museum of Art (New York)
Open Access Images - Searchable database of images. Most images also include descriptive information and a brief bibliography on the art and related works.
Heilbrunn Timeline of Art History - A useful source for placing an artist/work in historical context.
Museo del Prado (Madrid)
Explore the Collection - In addition to descriptions of the art works, each image includes a link to artist information, and multimedia resources.
National Gallery (London)
Search the Collection - Images may also include information about the artist and related works.
National Gallery of Art (Washington DC)
Search the Collection - Images may also include information about the artist and related works.
Philadelphia Museum of Art
Online Collection