php - How to add watermark on image before force download

Question

I want to add force download functionality with the help of

$this->load->helper('download');
$photo_path = "uploads/default/photos/".$photo;
$name = $photo_name.'.jpg';
$data = file_get_contents($photo_path); // Read the file's contents
$name = $photo_name.'.jpg';
force_download($name, $data); 

Now i want to add watermark image over the image before downloading the image.is this possible with image manipulation library or should i try to add watermark when uploading files.