code: def ReduceOpacity(im, opacity):
      
      assert opacity >= 0 and opacity <= 1
      if isalpha(im):
          im = im.copy()
      else:
          im = im.convert('RGBA')
  
      alpha = im.split()[3]
      alpha = ImageEnhance.Brightness(alpha).enhance(opacity)
      im.putalpha(alpha)
      return im
docs: Reduces Opacity.
  
      Returns an image with reduced opacity.
      Taken from http://aspn.activestate.com/ASPN/Cookbook/Python/Recipe/362879