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.

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