图片水平居中的问题，如题，如何让图片不管在什么分辨率下都在屏幕正中间

Question

<div id="div1" align="center" style="position:absolute;opacity:0.8;left:-1px; top:1px; width:100%; height:100%; background-color:#FFFFFF;z-index:1000;"> 
<div align="center">
<table>
<tr><td ><center><img src="image/screen/20130924.gif" height="100px" width="100px" /></center></td></tr>
</table>
</div>
</div>

Answer 1

<div style="position:absolute;left:50%;top:50%;margin-left:-50px;margin-top:-50px;">
<img src="image/screen/20130924.gif" height="100px" width="100px" />
</div>