Perhaps if you override it like this, it will randomly select the name of the array when calling the background image.
class Scene_Menu < Scene_Base
def create_menu_background
menu_bg_list = ['menu_bg_A', 'menu_bg_B', 'menu_bg_C']
@menu_background = Sprite.new
@menu_background.bitmap = Cache.system(menu_bg_list[Random.rand(menu_bg_list.length)])
end
end
