it does not work that way, try<script language="javascript">
var sWidth = 100;
</script>
<img src="http://www.csdn.net/images/csdn.gif" style="width:expression(sWidth);">
<input type="button" value="change" onclick="sWidth +=100;">
var sWidth = 100;
</script>
<img src="http://www.csdn.net/images/csdn.gif" style="width:expression(sWidth);">
<input type="button" value="change" onclick="sWidth +=100;">
var ww = 100;
</script><img src="http://www.csdn.net/images/csdn.gif" onload="this.width=ww">