REALball, http://koenig.h2m.ru/eg.php
$var = "uiojj [img]http://dsd.gif[/img] hjlkjll";
echo preg_replace("#\[img\](http:\/\/\S+[[:alnum:]].(bmp|png|jpg|gif))\[\/img\]#U", '<a href="$1"><img src="$1" alt="Нажми для просмотра" width="40" height="40" /></a>', $var);