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.

def get_or_add_image_part(self, image_file):
        """
        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.
        """
        image_part = self._package.get_or_add_image_part(image_file)
        rId = self.relate_to(image_part, RT.IMAGE)
        return image_part, rId