docs: def get_or_add_image(self, image_descriptor):
          """Return (rId, image) pair for image identified by *image_descriptor*.
  
          *rId* is the str key (often like "rId7") for the relationship between this story
          part and the image part, reused if already present, newly created if not.
          *image* is an |Image| instance providing access to the properties of the image,
          such as dimensions and image type.
          """
          image_part = self._package.get_or_add_image_part(image_descriptor)
          rId = self.relate_to(image_part, RT.IMAGE)
          return rId, image_part.image