Honestly I wouldn't think that is CSS so much as in inline style... In the default vBulletin postbit_legacy template style there is a line:
HTML Code:
<td class="alt2" width="175" style="border: $stylevar[cellspacing]px solid $stylevar[tborder_bgcolor]; border-top: 0px; border-bottom: 0px">
That is what controls that box in the postbit that you posted the picture of... the background image was probably added by altering this line. Find the name of the image then find the line that controls the main message area of the postbit:
HTML Code:
<td class="alt1" id="td_post_$post[postid]" style="border-$stylevar[right]: $stylevar[cellspacing]px solid $stylevar[tborder_bgcolor]">
Edit this line to add the background image to it, something like:
HTML Code:
<td class="alt1" id="td_post_$post[postid]" style="border-$stylevar[right]: $stylevar[cellspacing]px solid $stylevar[tborder_bgcolor]; background-image:url(images/misc/bg.gif)">