code: def get_or_add_image_part(self, image_file):
          
          image_part = self._package.get_or_add_image_part(image_file)
          rId = self.relate_to(image_part, RT.IMAGE)
          return image_part, rId
docs: Return an ``(image_part, rId)`` 2-tuple corresponding to an
          |ImagePart| object containing the image in *image_file*, and related
          to this slide with the key *rId*. If either the image part or
          relationship already exists, they are reused, otherwise they are
          newly created.