Frank Frazetta was an American illustrator and artist whos work has inspired so much of what we know today of the visual aspects of fantasy and some scifi. Even if you don't know some of his work you may have seen it or seen stuff inspired by it.
Michael Whelan is another great American illustrator hwo has work has been many book and album covers.
John Blanche is a British Illustrator who created many of the iconic images for the Warhammer 40K series. He is also doing book covers for other projects put if you point to any of the classic 90s and 80s images that are well known ultimaly creating the grimdark style.