Some submission guidelines for Gaming Art:

R. Talsorian Games, Inc.
Games: FUZION, Dragonball Z, Cyberpunk, Mekton, Bubblegum Crisis, Castle Falkenstein, TFOS

White Wolf
Games: Demon, Dark Ages, Exalted, Vampire, Werewolf, Mage, Hunter, Mind's Eye, Sword & Sorcery

Wizards of the Coast
Games: Star Wars, D&D, Call of Cthulhu

Steve Jackson Games
Games: GURPS, Car Wars, Illuminati