<?php
$cadena = "[img]http://www.google.com.ar/image.jpg|500|500[/img]";

if(preg_match_all('/\[img\](.*?)\[/img\]\/i', $cadena, $data)) {
  print_r($data);
 }
?>

Array
(
    [0] => Array
        (
            [0] => [img]http://www.google.com.ar/image.jpg|500|500
            [1] => http://www.google.com.ar/image.jpg
            [2] => 500
            [3] => 500
        )

)