One of the great things about the Web, is that it provides ready access to some fine quality content from all disciplines, including Art and Art History.
Since it's summer, I thought to share a few of my favorite sources of hi-resolution depictions with authoritative description.
Art History Resources is far and away the most definitive collection of Art links on the Worldwide Web. It's easy to navigate and will send you to more places than you could possibly need to go. So indulge and enjoy!
The Web Gallery of Art specializes in mostly European art of the past millenium, with a searchable database, biographies and guided tours of the great works according to style, period and museum. It's the next best thing to being there.
The Metropolitan Museum of Art may be the best comprehensive art museum in the world, and it's online collection, with over 400,000 freely accessible hi-res images is nearly as impressive.