Попробуйте это (адаптировано из sitepoint ):
<?php
header('content-type: image/jpeg');
$yourimagefile = 'something.jpg' // the image you're wanting to watermark
$watermark = imagecreatefrompng('watermark.png'); // the watermark
$watermark_width = imagesx($watermark);
$watermark_height = imagesy($watermark);
$image = imagecreatetruecolor($watermark_width, $watermark_height);
$image = imagecreatefromjpeg($yourimagefile);
$size = getimagesize($yourimagefile);
$dest_x = $size[0] - $watermark_width - 5;
$dest_y = $size[1] - $watermark_height - 5;
imagecopymerge($image, $watermark, $dest_x, $dest_y, 0, 0, $watermark_width, $watermark_height, 100);
imagejpeg($image);
imagedestroy($image);
imagedestroy($watermark);