Adding a watermark over a png (& jpg) image PHP

Question

So, I'm trying to add a watermark (png image) on jpg/png image. The thing is..

The code works if I upload a jpg, but when I upload a png the result is not the same as I want it to be.

If I upload a png the watermark will go in front of the image I uploaded, but then the background isn't transparant anymore (which is what I want).

if(isset($_GET['img'])) {
$image = $_GET['img'];

$stamp = imagecreatefrompng(DIR_PUBLIC . 'watermerk.png');
//$im = imagecreatefromjpeg(DIR_IMAGE . $image);

$fullPath = DIR_IMAGE . $image;

$path_parts = pathinfo($fullPath);
$ext = strtolower($path_parts["extension"]);

// Determine Content Type
switch ($ext) {
    case "png":
        $im = imagecreatefrompng($fullPath);
        $ctype="image/png";
        break;
    case "jpeg":
    case "jpg":
        $ctype="image/jpg";
        $im = imagecreatefromjpeg($fullPath);
        break;
    default:
        $ctype="application/force-download";
}

// Get dimensions
$imageWidth = imagesx($im);
$imageHeight = imagesy($im);

$logoWidth = imagesx($stamp);
$logoHeight = imagesy($stamp);

// Paste the logo
imagecopy(
// destination
    $im,
    // source
    $stamp,
    // destination x and y
    ($imageWidth - $logoWidth) / 2, ($imageHeight - $logoHeight) / 2,
    // source x and y
    0, 0,
    // width and height of the area of the source to copy
    $logoWidth, $logoHeight
);

// Output and free memory
  header("Content-Type: $ctype");
  imagepng($im);
  imagedestroy($im);
}

What I want: I want to be able to upload a png image and still have the watermark image over the image.