If you want "classic painter's" stuff - and if you know the name of an artist you like - one of the best places for good jpg images is Art Renewal Center. They're claiming 21,408 images today (2,466 Artists) - with a 50,000 image goal. I'd guess they run about 35% landscape, 35% portraits, and the rest "everything else," (lots of ships and harbors, too) so it helps to have an idea of someone to look for. You can also go direct to their Alphabetical Index of Artists (The A's).

The site "specializes" in "Academic Realism" - and you'll find no "modern/abstract/existential" stuff there - just good pictures. Google almost never goes there, and most of the stuff that Google image search comes up with are "inferior" quality images (IMHO).

If you want a broader search for "good quality" images, and again – if you know an artist's name or the "name" of a painting, you should try ArtCyclopedia. This is a "search engine site" that will direct you to where you can find stuff by any "recognized" artist.

The hardest part of finding good wallpaper pictures is that you need them 4 wide x 3 high to keep Windows from stretching them out of shape. (Well, you can always crop them.)

John