code: public function asImage($value, $options = [])
      {
          if ($value === null) {
              return $this->nullDisplay;
          }
  
          return Html::img($value, $options);
      }
docs: Formats the value as an image tag.
  @param mixed $value the value to be formatted.
  @param array $options the tag options in terms of name-value pairs. See [[Html::img()]].
  @return string the formatted result.