Return |Image| object containing poster frame for this movie.

        Returns |None| if this movie has no poster frame (uncommon).

def poster_frame(self):
        """Return |Image| object containing poster frame for this movie.

        Returns |None| if this movie has no poster frame (uncommon).
        """
        slide_part, rId = self.part, self._element.blip_rId
        if rId is None:
            return None
        return slide_part.get_image(rId)