'Return an (rId, image) 2-tuple for the image identified by
*image_descriptor*. *image* is an |Image| instance providing access
to the properties of the image, such as dimensions and image type.
*rId* is the key for the relationship between this document part and
the image part, reused if already present, newly created if not.'

def get_or_add_image(self, image_descriptor):

    image_part = self._package.image_parts.get_or_add_image_part(image_descriptor)
    rId = self.relate_to(image_part, RT.IMAGE)
    return (rId, image_part.image)
