Javascript控制图片间歇性轮换向上滚动

<script language="javascript" type="text/javascript">
var obj_marquee;
var marquee_spd = 4000;
var step_c = 0;
function marquee_init() {
obj_marquee = document.getElementById("marquee");
var obj_unit = obj_marquee.firstChild;
var n = Math.ceil(parseInt(obj_marquee.style.height) / obj_unit.offsetHeight);
for(var i=0; i<n; i++) obj_marquee.appendChild(obj_unit.cloneNode(true));
setInterval("step_c=0;setTimeout('marquee_show()',50)", marquee_spd);
return;
}
function marquee_show() {
var marquee_high = parseInt(obj_marquee.style.height);
var step = marquee_high/14;
if(obj_marquee.scrollTop >= obj_marquee.children[1].offsetTop) {
obj_marquee.scrollTop = 0;
marquee_show();
return;
}
obj_marquee.scrollTop += step;
if(step_c++ < 15) setTimeout("marquee_show()", 50);
return;
}
window.onload=marquee_init;
</script>

<div id="marquee" style="margin-top: 10px; overflow: hidden; margin: -2px 0 4px 0;
padding: 0; height: 32px; width: 100%; border: 0px; padding: 0px;" align="center">
<div style="border: 0px; padding: 0px">
<!-- Marquee Body Head -->
<div style="height: 32px; border: 0px; overflow: hidden;">
<a href="ad.html" target="_blank">
<img src="/images/2009052701.jpg" alt="" /></a>
</div>
<div style="height: 32px; overflow: hidden;">
<a href="ad2.html" target="_blank">
<img src="/images/2009052201.jpg" alt="" /></a>
</div>
<!-- Marquee Body Bottom -->
</div>
</div>