Check out these guidelines for using images from the World Wide Web...
More and more museums are making images available that are a decent size for projecting or printing. Usage is sometimes subject to agreeing to a 'Terms of Usage' agreement or signing up for a user account. Here are a few museum & library sites that have images available for download:
There are some other good sites on the web for finding art or art related images:
Additional sites are bookmarked on ColgateART's delicious page. Check it out here!