problem with imagecreate on new webserver

i am moving my webserver from a windows box to a linux box with php ver 5.2.0 and noticed i am having a problem with a php file(code below). i thought it was a module i was missing but looking at the php config file all the same modules are loaded. allow_url_fopen is enabled. what would i be missing in the php install that would make this code not work correctly from one machine to the other. when loading on the new box all i get is a image with 'http://localhost/imgfile.php' <? session_start(); define( 'SITE' , 'http://localhost' ); //define 7char code $md5 = md5(microtime() * mktime()); $string = substr($md5,0,7); //create image header("Content-type: image/png"); $img= imagecreatefrompng(SITE. '/images/img.png'); $white = imagecolorallocate($img, 233, 239, 239); $line = imagecolorallocate($img,233,239,239); for ($i = 1; $i <= 5; $i++) { $x1 = rand(1,149); $x2 = rand(1,149); $y1 = rand(1,49); $y2 = rand(1,49); imageline($img, $x1, $y1, $x2, $y2, $line); } //put code in image imagestring($img, 5, 35, 25, $string, $white); //save key to session $_SESSION['key'] = md5($string); imagepng($img); ?>