code: def _get_or_add_image(self, image_file):
          
          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
docs: Return an (rId, description, image_size) 3-tuple identifying the
          related image part containing *image_file* and describing the image.