启用 CORS 来解决这个问题（ajax跨域请求）

<input type="file" name="btn_Upload" value="上传" id="btn_Upload" />
<img src="" alt="" id="img_Upload" />

@section Scripts
{
    <link href="../../js/uploadifive-v1.2.2-standard/uploadifive.css" rel="stylesheet" />
    <script src="../../js/uploadifive-v1.2.2-standard/jquery.uploadifive.js"></script>
}

<script>
    $(function () {
        $('#btn_Upload').uploadifive({
            formData: { openId: '@ViewBag.OpenId' },
            multi: false,
            uploadScript: '@WXCommon.ConfigHelper.UploadUrl',
            onUploadComplete: function (file, data) {
                if (data) {
                    var json = $.parseJSON(data);
                    if (json.res) {
                        alert(json.msg);
                        $('#img_Upload').attr({
                            src: '@WXCommon.ConfigHelper.ThumbnailPath' + '/' + '@ViewBag.OpenId' + '/' + json.fileName
                        });
                    } else {
                        alert(json.msg);
                    }
                } else {
                    alert('服务器内部错误!');
                }
            }
        });
    });
</script>

//独立部署的图片服务器
public ActionResult Index()
{
    //if (Request.HttpMethod == "OPTION")
    //{
    //    Response.AddHeader("Access-Control-Allow-Methods", "GET,POST");
    //    Response.AddHeader("Access-Control-Allow-Headers", "Content-Type,Accept");
    //    Response.AddHeader("Access-Control-Max-Age", "1728000");
    //    Response.AddHeader("Access-Control-Allow-Origin", "*");
    //}
    string openId = Request["openId"];
    HttpPostedFileBase httpPostedFileBase = Request.Files["Filedata"];
    string fileName = httpPostedFileBase.FileName;
    string extensionName = System.IO.Path.GetExtension(fileName).ToLower();
    if (extensionName == ".jpg" || extensionName == ".jpeg")
    {
        string virtualPath = "/uploads" + "/" + fileName.Replace(extensionName, "") + DateTime.Now.ToString("yyyyMMddhhmmss") + extensionName;
        string fullPath = Server.MapPath(virtualPath);
        httpPostedFileBase.SaveAs(fullPath);

        string savePath = Server.MapPath("/thumbnails" + "/" + openId);
        string thumbnailFileName = Thumbnail.GetThumbnailImage(160, 120, fullPath, savePath, System.Drawing.Imaging.ImageFormat.Jpeg);
        //删除原始文件
        //System.IO.File.Delete(fullPath);
        NowModel model = new NowModel();
        model.Id = 0;
        model.OpenId = openId;
        model.FileName = thumbnailFileName;
        nowBLL.Insert(model);
        return Json(new { res = true, msg = "上传成功!", fileName = thumbnailFileName });
    }
    return Json(new { res = false, msg = "上传失败!" });
}

  <!--仅试用于IIS7以上版本，IIS6.0请用Response.AddHeader的方式-->
  <system.webServer>
    <validation validateIntegratedModeConfiguration="false" />
    <httpProtocol>
      <customHeaders>
        <add name="Access-Control-Allow-Origin" value="*"/>
        <add name="Access-Control-Allow-Methods" value="GET,POST,OPTION"/>
        <add name="Access-Control-Allow-Headers" value="Content-Type,soapaction"/>        
      </customHeaders>
    </httpProtocol>
  </system.webServer>