code: public function getThumbnails($videoId, $format = null)
      {
          $listResponse = $this->videos->listVideos('snippet', array('id' => $videoId));
          if (empty($listResponse)) {
              throw new \RuntimeException(sprintf('Could not find video with id %s', $videoId));
          }
          $video = $listResponse[0];
          $videoSnippet = $video['snippet'];
          if (is_null($format)) {
              return $videoSnippet['thumbnails']['data'];
          }
          if (!in_array($format, array('default', 'medium', 'high'))) {
              throw new \InvalidArgumentException(sprintf('Invalid format "%s"', $format));
          }
  
          return $videoSnippet['thumbnails']['data'][$format];
      }
docs: Get thumbnails of a video
  You can specify a format. If so, you get a single thumbnail of specified format,
  otherwise you get an array with all three available formats
  
  @param  string $videoId
  @param  string $format  "default", "medium", "high", or null
  @return array