Problem with transparent PNG

Hey.

First thank you for the script, very good.

But I have a problem when riding a png image on a jpg. Get an Error.


Catchable fatal error: Object of class D3Image could not be converted to string in /home/funny/public_html/blueimp/finaliza.php on line36




The Script is:

$marcoTopOn=new D3Image('file:'.$marcoTop.';');
$marcoTopOn->TurnOffBlending();

$image=new D3Image('width:'.$width.'px;height:'.$height.'px;background:#000;');
$image->BackGroundImage($url,'width:'.$width.';height:'.$height.';top:0px;left:0px;');
$image->add(new D3Image("file:".$marcoTopOn.';'), "top:0px;left:0px;");
$image->SaveToFile('testcartel.jpg');



And the error is in the line:
$image->add(new D3Image("file:".$marcoTopOn.';'), "top:0px;left:0px;");


Any idea that might be?

Thank you.

Greetings.
Juan