Some make record covers. Some make visual art and/or design. Some are sound artists or environmental scenarists. Some are also musicians, among other talents.

julass's list is incredibly useful: Visual Artists (link no longer working - please bring it back!)

eusebius also has a fine list; check it here.

Bili-Rubin has an excellent list of records made by visual artists, here.

french_75's list of illustrators is also pretty darn great: here.

Thanks to elchicofritto for his help, especially with the ones I'd never heard of :•)