JS跑马灯动不了

循环没停，但是图片也没动起来
demo.scrollLeft += demo2.offsetWidth; 的问题？
代码如下： 
<div id="demo" style="overflow:hidden;height:150px;width:500px;background:#214984;color:#ffffff; float:left;">
<table align="left" cellpadding="0" cellspace="0" border="0">
<tr>
<td id="demo1" valign="top">
<img src="./images/my_1.jpg" alt="" />
<img src="./images/my_13.jpg" alt="" />
<img src="./images/my_2.jpg" alt="" />
<img src="./images/my_8.jpg" alt="" />
<img src="./images/my_4.jpg" alt="" />
<img src="./images/my_9.jpg" alt="" />
<img src="./images/my_6.jpg" alt="" />
</td>
<td id="demo2" valign="top">
</td></tr>
</table></div><script type="text/jscript" language="javascript">
var speed=30;
var demo2=document.getElementById("demo2");
var demo1=document.getElementById("demo1");
var demo=document.getElementById("demo");
demo2.innerHTML=demo1.innerHTML;
demo.scrollLeft=demo.scrollWidth;
alert("hellow3");
function Marquee(){
if(demo.scrollLeft<=0)
{
 
 demo.scrollLeft += demo2.offsetWidth;
 }else{
demo.scrollLeft--;
alert("hellow");
}
}
var MyMar=setInterval(Marquee,speed)
demo.onmouseover=function() {clearInterval(MyMar)};
demo.onmouseout=function() {MyMar=setInterval(Marquee,speed)};
</script>