Legends of Arthur English 205
This database maintained by the Rochester University contains a large number of Arthurian images, mostly black and white illustrations, mostly nineteenth century, and in the public domain.
This site, maintained by David Ford of Binfield, England, also includes photographs of Arthurian sites; scroll down to the section on Dark Age Archaeology and Arthurian Tourism.
This is is probably the best serious (that is to say scholarly, responsible) site I have yet found on Celtic material on the Web. It allows you to view an extraordinary collection of celtic artifacts as well as sites (mostly in Ireland) and provides enormously useful timelines and maps.
Professor Sherman's class website has a nice collection of Irish art, which she invites Arthurian types to check out.
Students at Birmingham Southern College in Alabama have produced a wonderful virtual tour of Arthurian sites in Great Britain. This site includes photographs of many locations and artifacts associated with the Arthurian legend (including a variety of contenders for the title of "real" round table), links to primary texts, and descriptions of a great many Arthurian characters. If you should want to follow in the footsteps of these enterprising Arthurians, you can even take a look at the Ordnance Survey maps they used.
This commercial site allows you to view (and purchase reproductions of) the following Pre-Raphaelite paintings:
William Holman Hunt