[Frosty]

Are you using postbit or postbit_legacy? But I think it doesn't matter. Open the template of one you're using, and find this part:

PHP Code:

<if condition="$show['thumbnailattachment']">
<fieldset class="fieldset">
<legend>$vbphrase[attached_thumbnails]</legend>
<div style="padding:$stylevar[formspacer]px">
$post[thumbnailattachments]
</div>
</fieldset>
</if> 


Change it to:

PHP Code:

<if condition="$show['thumbnailattachment']">
<div style="padding:$stylevar[formspacer]px">
<center>$post[thumbnailattachments]</center>
</div>
</if> 


Save..

It looks like this:
http://i.imgur.com/iiCDS.png