DD_belatedPNG not working in dynamic HTML node

Question

everyone.

The enviroment is IE6.

There are 9 li node has png images, and random display 4 node, html code like this.

<ul>
    <li><img src="xxxxx.png"/></li>
    <li><img src="xxxxx.png"/></li>
    <li><img src="xxxxx.png"/></li>
    <li><img src="xxxxx.png"/></li>
    <li style="display:none;"><img src="xxxxx.png"/></li>
    <li style="display:none;"><img src="xxxxx.png"/></li>
    <li style="display:none;"><img src="xxxxx.png"/></li>
    <li style="display:none;"><img src="xxxxx.png"/></li>
    <li style="display:none;"><img src="xxxxx.png"/></li>
</ul>

I use DD_belatedPNG to display .png images, code like this:

<!--[if IE 6]>
<script type="text/javascript" src="/static/js/DD_belatedPNG.js"></script>
<script type="text/javascript">
try{ document.execCommand('BackgroundImageCache',false,true); }catch(e){ }
DD_belatedPNG.fix('*');
</script>
<![endif]-->

When the page onload, the .png image can display.

But when I use js to display the rest of others png images, something goes wrong. The png images can not be display.

How can I solve this problem. Thanks.