Добавить в файл functions.php :
add_filter('the_content', 'my_addlightboxrel');
function my_addlightboxrel($content) {
global $post;
$pattern ="/<a(.*?)href=('|\")(.*?).(bmp|gif|jpeg|jpg|png)('|\")(.*?)>/i";
$replacement = '<a$1href=$2$3.$4$5 rel="lightbox" title="'.$post->post_title.'"$6>';
$content = preg_replace($pattern, $replacement, $content);
return $content;
}
via wpbeginner.com
