As a visual medium, art historical research has benefited from the availability of high quality digital image databases. Some of the most popular include:
Art museum websites are an excellent place to find images of the artworks in their collections, or comparative images for other works of art you may be studying. Often, a museum may have digitized a substantial part of their collection and placed the images online.
Here are several large, useful collections (with image numbers as of July 2017):
Art history isn't all work and no play. In addition to the educational videos at the Khan Academy, feature films have also touched on fine art and artists. If you need a break, try watching one of these. Remember, though, that while it's always possible you might learn something, these are entertainment first: don't use them as reference material.
The Agony and the Ecstasy Michelangelo v. Pope Julius II (A scenery-chewing Charlton Heston v. Rex Harrison)
Nightwatching Rembrandt fiction, but stylish fiction