The Metropolitan Museum of Art's MetPublications site offers several catalogs of ancient Greek and Roman art. Note that some ancient Greek or Roman art is included in other collections that come up (such as one on Celtic art).
The Getty Museum has some exhibition catalogues available through their virtual library.
The British Museum's Library offers several thousand pages' worth of exhibition catalogs online. Search for the words "exhibition catalogue" (using the British spelling of "catalogue," and in quotation marks) for the most thorough results.
These very large and famous auction houses have websites that allow searches for both on sale and sold items. The images are good quality and some even allow closer examination of the items. Items include contemporary art, photography, fine art, and design as well as jewelry, antiquities, watches and clocks, European ceramics, and Asian and Chinese artwork and calligraphy.