code: def _poster_frame_rId(self):
          
          _, poster_frame_rId = self._slide_part.get_or_add_image_part(
              self._poster_frame_image_file
          )
          return poster_frame_rId
docs: Return the rId of relationship to poster frame image.
  
          The poster frame is the image used to represent the video before it's
          played.