list($width, $height) = getimagesize($filename);
// thumbnail
$new_width = 160;
$new_height = 120;
// Resample thumbnail
$image_p = imagecreatetruecolor($new_width, $new_height);
$image = imagecreatefromjpeg($filename);
imagecopyresampled($image_p, $image, 0, 0, 0, 0, $new_width, $new_height, $width, $height);
// Output
imagejpeg($image_p, $filename2, 80);
Den PHP-Code hatte ich im Netz gefunden.