Everyone knows Ralph McQuarrie for his Star Wars concept art, but he did cover art as well: Here’s a 1986 cover by McQuarrie for Eric Frank Russell’s Sinister Barrier, via Ragged Claws
The HR Giger guitar
Cover art by John Schoenherr
H.R. Van Dongen
Don Brautigam cover art for Up the Walls of the World, by James Tiptree, Jr., 1978.