imagecopy transparency

Hi all, i have written a simple php script to resize images from csv file and watermark them, the problem is if the watermark is a transparent png then all of the background is transparent not just the image, so if the backgroundcolour is blue then i get a blue rectangle with the watermark text, any ideas

	 //watermark
  $logoImage = "$watermarkRoute";
  $logoExtension = strtolower(substr(strrchr($logoImage,'.'),1));
  if($logoExtension == "jpg") {
  $logo = imagecreatefromjpeg($logoImage);
  }
  if($logoExtension == "png") {
   $logo = imagecreatefrompng($logoImage);
  }

  $watermarkImageWidth_src = imagesx($logo);
	 $watermarkImageheight_src = imagesy($logo);

	 imagecopy($img, $logo, 20, 20, 0, 0, $watermarkImageWidth_src, $watermarkImageheight_src);

Thanks