Find this line in postbit_legacy template:
HTML Code:
<td class="alt2" width="175" style="border: $stylevar[cellspacing]px solid $stylevar[tborder_bgcolor]; border-top: 0px; border-bottom: 0px">
Edit it similer to this:
HTML Code:
<td class="alt2" width="175" style="border: $stylevar[cellspacing]px solid $stylevar[tborder_bgcolor]; border-top: 0px; border-bottom: 0px; background: url(background-image.png) no-repeat bottom left">
.