Just wrap a div around $layout in the template and in the Layout CCS and give the div a background image. Or if there is a div already around it - just give that a background image.
If your content area expands ie. height: auto; give the div a background colour the same as the background to the image

Yes you need to put the background image in the CSS not the SS

.SS looks like

<div id="Main" class="typography">
$Layout
</div>

CSS (themes/YOURTHEME/css/layout.css) could look like

#Main {
height: auto;
width: ????px;
padding: 10px;
float: left;
background: #FFFFFF url(../images/bglogo.gif) no-repeat;
}