imagecreatefrompng image ontop of base image using imagestring

Question

Okay, I want to have a Base image (background.png) 400x400

And An Avatar, 200x200 and put it ontop of the background.png in imagecreatefrompng using imagestring

have: http://puu.sh/5KEAG.png

want: http://puu.sh/5KEF4.png

code so far;

<?php
$image = ('background.png');

$im = imagecreatefrompng($image);

$white = imagecolorallocate($im, 255, 255, 255);
$width = imagesx($im);
$height = imagesy($im);

//where I want image avatar.png 200x200

Header('Content-type: image/png');
imagepng($im);
imagedestroy($im);
?>