[Plugin: AutoThumb] Autothumb and Image Overlays

I am trying to use a filter to overlay an image over another.

<?php echo getphpthumburl($image[0], “w=300&h=300&zc=1&fltr[]=over|”.$overlay_url.”|0|0|100″)?>

$overlay_url is http://domain.name/wp-content/themes/themename/images/overlay.png

http://wordpress.org/extend/plugins/autothumb/