Download and phpThumb from http://phpthumb.sourceforge.net/ and extract to vendors/phpthumb
Usage :
App::import('Component', 'Phpthumb.Phpthumb');
$Thumbnail = new PhpthumbComponent;
$data = $this->PhpThumb->create('crop', '50x100');
$cache = '/path/to/save/as/cache/image/file/filename.ext';
$fp = fopen($cache, 'w');
fwrite($fp, $data);
fclose($fp);
chmod($cache, 0777);