http://www.oir.ucf.edu/louvre/paint/auth/dali/
http://www.oir.ucf.edu/louvre/paint/auth/escher/
The Web Louvre has a bunch of mirror sites. If you aren't in the USA,
when you try to access these URLs you'll get a list of sites all
around the world. Once you're in, it's very easy to find paintings by
a given artist.
Cheers,
Brandon