Transpareter ImageColorAllocate Wert?

header("Content-type: image/png");  
$email    =    $Mail;
$length    =  (strlen($email)*8);
$im = @ImageCreate ($length, 20)
     or die ("Kann keinen neuen GD-Bild-Stream erzeugen");  
$background_color = ImageColorAllocate ($im, 202, 202, 202); // White: 255,255,255 (Background-Color)
$text_color = ImageColorAllocate ($im, black, black, black);
imagestring($im, 3,5,2,$email, $text_color);
imagepng ($im);
...