For a variety of reasons no web image database has it all. Plus, these databases often don't provide the usability that the imagery and the text surrounding the images can provide in their native databases.

In the past few weeks, we've started to collect direct links to some of the database of paintings, sculptures, etc that some of the great art museums around the world hold. We are only in the early stages.

You can find the links we've collected so far here:
http://www.resourceshelf.com/category/arts/art-museums/

We do our best to update with new databases about once a week.

Btw, one database of special interest from a search perspective can be found at The Hermitage.

Their database offers IBM's QBIC (Query By Image Content) technology that has been available for 10 years. Search for similar holdings by color or layout. More about QBIC here.
http://domino.research.ibm.com/comm/pr.nsf/pages/rsc.qbic.html
and
http://wwwqbic.almaden.ibm.com/