public function Watermark(GD $gd) {
   // Read the watermark picture
   $watermark = imagecreatefrompng($URL . '/themes/Ace/images/watermark.png');
   $watermark_width = imagesx($watermark);
   $watermark_height = imagesy($watermark);

   // Read Full size Silverstripe image
   $imageToWatermark = imagecreatefromjpeg($this->URL);

   // Calculate sizes
   $size = getimagesize($this->URL);
   $dest_x = $size[0] - $watermark_width - 5;
   $dest_y = $size[1] - $watermark_height - 5;

   // Do transformations and save image in assets
   imagecopymerge($imageToWatermark, $watermark, $dest_x, $dest_y, 0, 0, $watermark_width, $watermark_height, 40);
   imagepng($imageToWatermark,$webroot . $this->filename . "_watermarked.png");
   
   // And this is the trick ;-) Create an object Silverstripe can work with....
   $GDoutput= new GD($webroot . $this->filename . "_watermarked.png");
   return $GDoutput;
   }