@manipulative
    def watermark(self, image, transparency=0.0, left=0, top=0):
        with image.clone() as watermark_image:
            watermark_image.transparentize(transparency)
            self.composite(watermark_image, left=left, top=top)
        self.raise_exception()
Transparentized the supplied ``image`` and places it over the
dataset/ETHPy150Open dahlia/wand/wand/image.py/BaseImage.watermark