loading 卡住不动

this.Button1.Attributes.Add("onclick","javascript:document.getElementById('loading').style.visibility='visible';");

<div id="loading" runat="server" style="Z-INDEX: 12000; FILTER: Alpha(Opacity=80); LEFT: 0px; WIDTH: 100%; CURSOR: wait; POSITION: absolute; TOP: 0px; HEIGHT: 100%; BACKGROUND-COLOR: #cccccc">
<table width="100%" height="100%">
<tr align="center" valign="middle">
<td>
<table width="350" height="135" border="0" cellpadding="0" cellspacing="0" style="FILTER: Alpha(Opacity=100); WIDTH: 350px; HEIGHT: 135px">
<tr>
<td height="28"><img src="images/login_loading_top.gif" width="350" height="28"></td>
</tr>
<tr>
<td background="images/login_loading_bg.gif"><div align="center"><img src="images/ajax-loader.gif" width="16" height="16"></div>
</td>

</tr>
</table>
</td>
</tr>
</table>
</div>


可是提交的时候loading里的图片卡住不动，怎么回事，谢谢！
--------------------编程问答-------------------- 请大家帮忙看看，怎么回事，谢谢

补充：.NET技术 ,  ASP.NET