Depends on how you want to go about it...
1) Set the opacity to 0, ans set the contents to your desired bitmap
class Window_Whatever < Window_Base
def initialize
super(x, y, width, height)
self.opacity = 0
self.contents = RPG::Cache.folder('name').dup
# ...
2) Create a sprite in the initialize method
class Window_Whatever < Window_Base
def initialize
super(x, y, width, height)
self.opacity = 0
@sprite = Sprite.new(Viewport.new(x, y, width, height))
@sprite.bitmap = RPG::Cache.folder('name')
# ...[/code]