1.添加javascript代码

<script type="text/javascript" language="javascript">
function Preview(imgFile)
{
    document.getElementById("pic").filters.item("DXImageTransform.Microsoft.AlphaImageLoader").src = imgFile.value;

}
</script>

2.FileUpload控件添加

<asp:FileUpload runat="server" ID="imgFile" onchange="javascript:Preview(this);" />

3.预览图显示

<div id="pic" style="filter:progid:DXImageTransform.Microsoft.AlphaImageLoader(sizingMethod=scale);width:400px;height:300px;"></div>