imagecopyresized virker ikke

prøver lige igen :)

hvad er der galt her?!

<?
session_start();

require('../req/functions.req');

header("content-type: image/jpeg");

$width = 100;
$height = 120;

$image_src = imagecreatefromjpeg($DIR_USER_PIC.$_GET['user_id'].'.jpg');
$width_src = imagesx($image_src);
$height_src = imagesy($image_src);

$thumb = imagecreatetruecolor($width, $height);
imagecopyresized($thumb, $image_src, 0, 0, 0, 0, $width, $height, $width_src, $height_src);

imagejpeg($thumb);
imagedestroy($thumb);
?>