I've got an improved postbit display too.
Instead of modiying the postbit, just have a cup of tea

.
And then open template postbit_signatureimg and replace the entire contents with:
<img src="$signatureurl" border="0" alt=""><br>
Then open the template postbit_signature and replace the entire contents with:
<p><normalfont>__________________<br>
$post[signatureimg]
$post[signature]</normalfont></p>
Now it will look a lot neater