Google Art Project is a fascinating web site that has photos of art from museums from all around the world.
The pictures are very high quality and you can even zoom in. Besides viewing the artwork, you can also take a virtual tour of the museum itself. The virtual tour reminds me of Google Street View.
There is a "visitor's guide" for the museums and floor plans to help you navigate. The artwork pictures are incredible quality and resolution. It is truly amazing.
Each piece of artwork has in information tab on the right that has the type of painting, artist, size, audio commentary on the piece, viewing notes, history and more.
You can even create your own "collections" of art.
This is a great way to experience and view great artwork without leaving your home or classroom and is also a "art classroom" on the web.
Here's an example of one of the pieces of artwork. "Salisbury Cathedral from the Bishop's Garden"