Return the rId of relationship to poster frame image.

        The poster frame is the image used to represent the video before it's
        played.

def _poster_frame_rId(self):
        """Return the rId of relationship to poster frame image.

        The poster frame is the image used to represent the video before it's
        played.
        """
        _, poster_frame_rId = self._slide_part.get_or_add_image_part(
            self._poster_frame_image_file
        )
        return poster_frame_rId