Another (possibly slightly hackier, but still does the same) way to do this only with out the GD libs, would be something like this:
PHP Code:
<?php
$filename = sprintf ("sig%d.png", rand (0, 9));
readfile($filename);
?>
Rather than creating a new image, this would just 'dump' the contents of the image, keeping the image headers in tact.
The only main differences between this and the GD version are (like i said), this doesnt require GD, and it can be generated faster, thru not having to create a seperate image.