Today's old old-school artist is Frank C. Papé, an English-born artist and illustrator, better known around these OSR-blog parts as the guy who illustrated a version of The Odyssey as well as Siegfried and Kriemhild. While the illustrations from both those books are absolutely beatiful, they are a bit too extravagant (many in color) and detailed to give them that "old-school" RPG feel that I dig so much.
One of the reasons I like to look through the children's portion of the "vintage/nostalgia" section of Half-price Books, is because most of the books have lots of pictures. Now, you can either assume that's because I'm an artist looking for reference/inspiration, or because I'm looking for pictures because I ain't so good with words. In truth, it's because I stumble upon cool images from the likes of Frank C. Papé. The illustrations below are from Uncle Ray's Story of the Stone-age People, an account of the European caveman. And these illustrations seem moderately reminiscent (though a precursor to) Alan Hunter's Fiend Folio illustrations.