<?php

$string = '[url=http://d...content-available-to-author-only...n.com]My Webpage[/url][img]http://d...content-available-to-author-only...n.com/image1.jpg[/img][center][img]http://d...content-available-to-author-only...n.com/image2.gif[/img][/center][center][thumb]http://d...content-available-to-author-only...n.com/image3.png[/thumb][/center]';

preg_match_all("@\[(img|thumb)\](.+?)\[/\\1@", $string, $m); print_r($m);