I have a working PHP image / text stamping script, but i need a little help. Example shown please help.?

Ok the below script inserts the text in "$string" onto the image "EM1.png". This works great. But say if instead of wanting to insert text onto the image i wanted to insert another smaller image on the EM1.png what would i do?

Bascially i want to stamp a small picture onto images like a logo to stop people stealing them.

<?php

header("Content-type: image/png");

$string = "super text";

$im = imagecreatefrompng("Empty/EM1.png");

$orange = imagecolorallocate($im, 20, 210, 60);

$px = (imagesx($im) - 7.5 * strlen($string)) / 2;

imagestring($im, 3, $px, 56, $string, $orange);

imagepng($im);

imagedestroy($im);

?>