The web is a wonderful source of images by famous artists, and websites providing information about artwork, and about famous museums around the world.
One of the best resources that I have found on the web is done by Chris Witcombe, an Art History professor at Virginia's Sweet Briar College. He has made the most comprehensive site I have seen, containing links to almost every major art museum around the world, as well as links to art images on the web, biographical information about artists, and the history of art in every culture, from every era.
Another excellent college source is from the Fine Arts Department at Boston College. Boston College's Digital Archive of Art, by Art History Professor Jeffery Howe , has a wonderful page with links to slides done by Professor Howe (you may also recognize their Art on the Web logo, which you see at the top of this page).
Below is a selection of some of my favorite paintings. Click on any one of them to see a full-page display and description.