I am sure that Art Renewal Center has been mentioned in the forums before as a great online resource but I wanted to give it a mention in its own topic. It has over 50 000 high quality images and the top artists of the site can be found here:

http://www.artrenewal.org/articles/2005/Reasons/museum1.asp

and the homepage:

http://www.artrenewal.org/

Enjoy,

-Noah