PHP : Watermark

<?php

watermark('img.png', 'watermark.png');

function watermark($dst_im, $src_im, $im_save=NULL){
    $src_im = imagecreatefrompng($src_im);//create image from source
    $dst_im = imagecreatefrompng($dst_im);//create image from destination
    imagecopy($dst_im, $src_im, 0, 0, 0, 0, imagesx($src_im), imagesy($src_im));//merge watermark into image
    header('Content-type: image/png');//send image header to browser
    imagepng($dst_im);//send image content to browser
}
?>

Sumber : http://daif.net/script/watermark.phps