js代码：

<script type="text/javascript">
//下面用于图片上传预览功能
function setImagePreview() {
var docObj = document.getElementById("fileImg");

var imgObjPreview = document.getElementById("preview");
if (docObj.files && docObj.files[0]) {
//火狐下，直接设img属性
imgObjPreview.style.display = 'block';
imgObjPreview.style.width = '150px';
imgObjPreview.style.height = '180px';
//imgObjPreview.src = docObj.files[0].getAsDataURL();

//火狐7以上版本不能用上面的getAsDataURL()方式获取，需要一下方式
imgObjPreview.src = window.URL.createObjectURL(docObj.files[0]);
}
else {
//IE下，使用滤镜
docObj.select();
var imgSrc = document.selection.createRange().text;
var localImagId = document.getElementById("localImag");
//必须设置初始大小
localImagId.style.width = "150px";
localImagId.style.height = "180px";
//图片异常的捕捉，防止用户修改后缀来伪造图片
try {
localImagId.style.filter = "progid:DXImageTransform.Microsoft.AlphaImageLoader(sizingMethod=scale)";
localImagId.filters.item("DXImageTransform.Microsoft.AlphaImageLoader").src = imgSrc;
}
catch (e) {
alert("您上传的图片格式不正确，请重新选择!");
return false;
}
imgObjPreview.style.display = 'none';
document.selection.empty();
}
}

Html界面

<div id="localImag">
<img id="preview" src="" width="150" height="180" style="display: block; width: 150px; height: 180px;">
</div>
<asp:FileUpload ID="fileImg" runat="server" CssClass="fileImg" onchange="javascript:setImagePreview();" />

测试：谷歌浏览器和IE浏览器没有问题！