public function getThumbnail($presentationId, $pageObjectId, $optParams = array())
  {
    $params = array('presentationId' => $presentationId, 'pageObjectId' => $pageObjectId);
    $params = array_merge($params, $optParams);
    return $this->call('getThumbnail', array($params), "Google_Service_Slides_Thumbnail");
  }