I used this to watermark images. This is the function I wrote:

<?php
function watermark($url,$logo){
    $bwidth  = imagesx($url);
    $bheight = imagesy($url);
    $lwidth  = imagesx($logo);
    $lheight = imagesy($logo);
    $src_x = $bwidth - ($lwidth + 5);
    $src_y = $bheight - ($lheight + 5);
    ImageAlphaBlending($url, true);
    ImageCopy($url,$logo,$src_x,$src_y,0,0,$lwidth,$lheight);
}
?>

Usage:
<?php
//$current_image would be your image the watermark is overlayed onto. Make sure it's imagecreatefrom*** to work.
watermark($current_image,$watermark_image);
?>

Hope this helps someone.