Uložení obrázku z funkce

<?php
function watermark($im){
   $watermark = imagecreatefrompng('wm.png');
   imagealphablending($watermark, false);
   imagesavealpha($watermark, true);
   $watermark_width = imagesx($watermark);
   $watermark_height = imagesy($watermark);
   $image = imagecreatetruecolor($watermark_width, $watermark_height);
   
   $image = imagecreatefromjpeg($im);   
   $size = getimagesize($im);
   
   $dest_x = ($size[0] - $watermark_width)/2;
   $dest_y = ($size[1] - $watermark_height)/2;

   imagecopy($image, $watermark, $dest_x, $dest_y, 0, 0, $watermark_width, $watermark_height);

   header("content-type:image/jpeg");
   imagejpeg($image);
}

watermark("test.jpeg");
?>