Using PHP GD library to adding watermark is as simple as merging 2 images.

First, we create gd image from photo to add watermark to:

$bg = imagecreatefromjpeg('photo.jpg');

Next, we create gd image from watermark file (logo.png) and get it's size:

$wm = imagecreatefrompng('logo.png');
$wm_size = getimagesize('logo.png');

Now place watermark at 25 pixels from top & left borders:

imagecopy($bg, $wm, 25, 25, 0, 0, $wm_size[0], $wm_size[1]);

Finally, save resulting JPG image to result.jpg:

imagejpeg($bg, 'result.jpg');

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