Artstor is an online repository of more than a million digital images relating to art history, covering a wide range of time periods and cultures. This is the major database for images of art objects. Thumbnail images from Artstor are included in ArticlesPlus, with links to the full records in Artstor itself.
Flickr is a vast photo-sharing site. You can search the whole site and/or explore some of the sections listed below (originally posted at AWOL: the Ancient World Online) which are devoted explicity to the ancient world.