Animating Bitmap in custom window

[darkspot]

i'm trying to create a window with a picture inside it that slightly moves up and down (as if floating)

This is what i've got so far, but I can't for the life of me figure out how to make that pic move

module DrawWindow 

    def draw_window()
    $window = Window_Base.new(0,296, Graphics.width, 120)
    bitmap = Bitmap.new("Graphics/Pictures/Face.png")
    $window.contents.blt(0,0, bitmap, Rect.new(0,0,bitmap.width,bitmap.height))
  end
 
  def draw_message(string)
    
    $window.draw_text(112,14, Graphics.width, 50, $txt,0)

  end
 
    def clear_window()
    $window.dispose
  end
end