<style>
#idDiv{width:80%;height:300px;padding:6px;background-color:buttonshadow;background-image:url(1.jpg);background-repeat:no-repeat;color:#FFFFFF;}
#idCodeDiv{width:80%;padding:4px;font-family:verdana,tahoma;margin:12px 0px 0px 0px;background-color:#EEEEEE;font-weight:bold;}
</style>
<script>
function rdl_change(e){
if document.all("idSel").selectedIndex!=0{
document.all("idDiv").style.backgroundPositionX=document.all("idSel").options[selectIndex].value
}
}
</script>

<div id=idDiv>请从下方选择背景图的位置。</div>
<br>
<select id="idSel" onchange="rdl_change();">
<option value="null">---请选择---
<option value="center">center
<option value="left">left
<option value="right">right
<option value="20px">20px
<option value="90%">90%
</select>
<br>
<div id=idCodeDiv>background-position-x : %0 ;</div>