<game_actor>.set_graphic(character_name, character_hue, battler_name, battler_hue)
Replace <game_actor> with $game_actors[actor_id] or $game_party.actors[index].
Replace character_name, character_hue, battler_name, battler_hue with
'filename', 0 to 360, 'filename', 0 to 360