Return an (rId, description, image_size) 3-tuple identifying the
        related image part containing *image_file* and describing the image.

def _get_or_add_image(self, image_file):
        """
        Return an (rId, description, image_size) 3-tuple identifying the
        related image part containing *image_file* and describing the image.
        """
        image_part, rId = self.part.get_or_add_image_part(image_file)
        desc, image_size = image_part.desc, image_part._px_size
        return rId, desc, image_size