Code:
imagettftext($img, $fontsize, $left, (($i * $zeilenabstand) - $zeilenabstand + $top), $text_color, $output); // LINE2
This line here is missing the vital bit of code ...
Code:
imagettftext($img, $fontsize, $left, (($i * $zeilenabstand) - $zeilenabstand + $top), $text_color, "verdana.ttf", $output); // LINE2
You need to pick a TTF font file, upload it to the same folder as your signature.php file and specify the filename of that font here ... see?