Нужно зделать смену background у определенного div основа такая
CSS элемента у каторго нужно сменить в данном случае это
cont-bot
#casing{margin:0;padding:0;background:#f3f3f3 url('/images/casing.png') -1209px 0 repeat-y;}
.cont-top {background:url('/images/casing.png') 0 0 no-repeat;}
.cont-bot {background:url('/images/casing.png') right bottom no-repeat;padding:20px 20px 360px 20px;}
<html xmlns="http://www.w3.org/1999/xhtml">
<head profile="http://gmpg.org/xfn/11">
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
<link type="text/css" rel="StyleSheet" href="/_st/my.css" />
</head>
<body>
<div id="overlay">
<div class="wrapper">
$GLOBAL_AHEADER$
<div class="clr"></div>
<div id="casing">
<div class="cont-top"><div class="cont-bot">
<!-- <middle> -->
<div class="left">
$GLOBAL_CLEFTER$
</div>
<div id="content">
<!-- <body> -->$CONTENT$<!-- </body> -->
</div>
<div class="right">
$GLOBAL_DRIGHTER$
</div>
<!-- </middle> -->
<div class="clr"></div>
</div></div>
</div>
</div>
<div id="footer">
<div class="wrapper">
<div class="fleft">$GLOBAL_BFOOTER$</div>
<div class="fright">
</div>
<div class="clr"></div>
</div>
</div>
</div>
<script type="text/javascript" src="$HOME_PAGE_LINK$js/cufon.js"></script>
<script type="text/javascript" src="$HOME_PAGE_LINK$js/font.js"></script>
<script type="text/javascript" src="$HOME_PAGE_LINK$js/ui.js"></script>
</body>
</html>