This artwork was by Tom Barber.
Jimster has done some research and found that he was the illustrator on two of Harlan Ellison's stories in Heavy Metal: "How's The Nightlife On Cissalda" (Nov. 77) and "Flop Sweat (Mar. 79); his art as appearing on the Tros of Samothrace paperbacks that came out in the late 70's (by Talbot Mundy) as well as the covers for Adrian Cole's 'Dreamlords' cycle. He also did a series of Weird Tales antholgies.
Andrew J. Offutt wrote Sign of the Moonbow with permission of Glenn Lord from Robert E Howard's character Cormac Mac Art.