in alternativa puoi usare questo resize.php che ti mantiene costanti le proporzioni senza sfigurarti l'immagine:

codice:
<?
function resize_image($img_res, $maxX, $maxY) {
  $actualX=imagesx($img_res);
  $actualY=imagesy($img_res);
		
  if ($actualY>$maxY){
    $newY = $maxY;
    $newX = ($actualX/$actualY)*$newY;
    if ($newX>$maxX)
      $newX = $maxX; $newY = ($actualY/$actualX)*$newX;
  }elseif ($actualX>$maxX){
    $newX = $maxX;
    $newY = ($actualY/$actualX)*$newX;
    if ($newY>$maxY)
      $newY = $maxY; $newX=($actualX/$actualY)*$newY;
  }else{
    $newX = $actualX;
    $newY = $actualY;
  }
  $tmp_img_res = imagecreatetruecolor($newX, $newY);
		
  $res = imagecopyresampled($tmp_img_res, $img_res, 0, 0, 0, 0, $newX, $newY, $actualX, $actualY);

  return $tmp_img_res;
}

function save_image($img_res, $filename, $quality=100) {
  imagejpeg($img_res, $filename, $quality);
}

function print_image($img_res, $quality=100) {
  header('Content-Type: image/jpeg');
  imagejpeg($img_res,NULL, $quality);
}
?>