For an organized view of all art everywhere, go to the Gateway to Art History (http://www.harbrace.com/art/gardner/), which is designed to be used with the monumental Gardner's Art through the Ages. If you're really serious about art, this is the book to have ($87 at http://www.amazon.com/exec/obidos/ASIN/0155011413/), but the site is extremely useful even without the book.
For links to thousands of Internet art web sites, use ART HISTORY RESOURCES (http://witcombe.sbc.edu/ARTHLinks.html), one section of which has more links to Ancient Roman art than anyone has a right to know (http://witcombe.sbc.edu/ARTHancient.html#Roman).
Orazio Centaro claims that his OCAIW (http://www.ocaiw.com/indexing.htm) is the most complete catalogue of art images on the web, and he may be right -- who can count them? Or maybe Carol Gerten's CGFA (http://sunsite.auc.dk/cgfa/featured.htm) is bigger.
EuroWeb's Web Gallery of Art (http://gallery.euroweb.hu/index1.html)
has more than 6000 European works (1150-1750 AD) many of which have commentary
and artist biographies.