Maybe try adding the stuff in red in your postbit template:
Code:
<tr>
<td class="alt2" style="padding:0px; background url(backgroundimage.jpg);">
<!-- user info -->
Whoops, just read it's postbit_legacy (this is from mine, your may be different):
Code:
<tr valign="top">
<td class="alt2" width="175" style="border: $stylevar[cellspacing]px solid $stylevar[tborder_bgcolor]; border-top: 0px; border-bottom: 0px; background url(backgroundimage.jpg);">