The British Museum website offers an image database with high resolution images for downloading and interactive features to accompany their exhibitions.
C4 Contemporary Art Database is a catalog of contemporary artists, which includes artists' biographies, an explanation of their relevance, examples of work and relationships to other artists. The site is still being developed, but new resources are added daily.
The Getty Search Gateway allows users to search over 10,000
high-resolution images across several of the Getty repositories,
including collections databases, library catalogs, collection
inventories, and archival finding aids. Users can use the tool to
browse the collections broadly, by type of resource (i.e., paintings,
books, archives), or to search directly for assets related to very
specific subjects—historical periods, artists, object types, etc. 
The Metropolitan Museum of Art's Heilbrunn Timeline of Art History is funded by the Heilbrunn Foundation, New Tamarind Foundation, and Zodiac Fund. The Timeline of Art History presents the Met’s collection via a chronological, geographical, and thematic exploration of global art history. The Timeline comprises 300 timelines, 930 essays, close to 7,000 objects, and a robust index, and is regularly updated and enriched to provide new scholarship and insights on the collection.
The LACMA Collection digital database contains images of works ranging from 10,000 BCE to the twenty first century.
The Louvre's Atlas: Database of exhibits allows the direct online consultation of 35,000 works of art
exhibited in the Louvre. Online visitors can access the basic
information displayed on labels accompanying works in the museum,
together with authoritative commentary and analysis by the curators and
staff.
Use the Smithsonian's SIRIS (Smithsonian Research Information System) to search over 7.4 million records, with 568,100 images, video and sound files from Smithsonian museums, archives, and libraries.
VADS is a visual art collection comprising over 100,000 images that can be downloaded for instructional use and research purposes. 