image-size php script wont work with my theme

hi,
when i paste this php-script into my single.php
it won’t work…
why?…as standalone it works perfect

<?php
// File and new size
$filename = 'http://www.multistorey.net/images/themselves2.jpg';
$percent = 0.5;

// Content type
header('Content-type: image/jpeg');

// Get new sizes
list($width, $height) = getimagesize($filename);
$newwidth = $width * $percent;
$newheight = $height * $percent;

// Load
$thumb = imagecreatetruecolor($newwidth, $newheight);
$source = imagecreatefromjpeg($filename);

// Resize
imagecopyresized($thumb, $source, 0, 0, 0, 0, $newwidth, $newheight, $width, $height);

// Output
imagejpeg($thumb);
?>