public function addImage( $key, $location, $caption = '', $geolocation = '', $title = '', $license = '' )
    {
        /**
         * @var \Lti\Sitemap\SitemapUrl $node
         */
        $node = $this->get( $key );
        if ( ! is_null( $node )) {
            $node->addImage( new SitemapImage( $location, $caption, $geolocation, $title, $license ) );
            $this->hasImages = true;
        }
    }