You`ll need to do a little bit more hacking that just bottom aligining the sig.. in your postbit find:
<td bgcolor="$post[backcolor]" width="100%" valign="top">
<smallfont>$post[icon] <b>$post[title]</b></smallfont>
<p><normalfont>$post[message]</normalfont></p>
$post[attachment]
<p>$post[signature]</p>
<p>$post[editedby]</p>
<p align="right"><smallfont><a href="report.php?s=$session[sessionhash]&postid=$post[postid]">Report this post to a moderator</a> | $post[iplogged]</smallfont></p>
</td>
Replace that entire section with:
<td bgcolor="$post[backcolor]" width="100%" height="100%" valign="top">
<table width="100%" border="0" cellpadding="0" align="left" height="100%">
<tr>
<td bgcolor="$post[backcolor]" colspan="2" align="left" valign="top" height="20"><smallfont>$post[icon] <b>$post[title]</b></smallfont></td>
</tr>
<tr>
<td bgcolor="$post[backcolor]" colspan="2" align="left" valign="top"><p><normalfont>$post[message]</normalfont></p></td>
</tr>
<tr>
<td bgcolor="$post[backcolor]" colspan="2" align="left" valign="bottom" height="20"> $post[attachment]</td>
</tr>
<tr>
<td bgcolor="$post[backcolor]" colspan="2" align="left" valign="bottom" >$post[signature]</td>
</tr>
<tr>
<td bgcolor="$post[backcolor]" align="left" valign="top" height="8">$post[editedby]</td>
<td bgcolor="$post[backcolor]" align="right" valign="top" height="8"><smallfont><a href="report.php?s=$session[sessionhash]&postid=$post[postid]">Report this post to a moderator</a> | $post[iplogged]</smallfont></td>
</tr>
</table>
</td>
that should then align everything nicely in your postbit :)