function upload() {
            VAR options = {
                tyPE: "POST",                            //当然这个是传送方式
                url: '../Include/Files.ashx',        //一般处理程序的路径
                success: function (msg) {        //返回的参数
                    $("#server_img").attr("src", msg);            //回显图片。
                }
            };
            // 将options传给ajaxForm
            $('#aspnetForm').ajaxSubmIT(options);
 }

 public void PRocessRequest(HttpContext context)
    {
        HttpFileCollection files = context.Request.Files;              // From中获取文件对象
        if (files.Count > 0)
        {
             string path = "";                                                            //路径字符串
            Random rnd = new Random();
            for (int i = 0; i < files.Count; i++)
            {
                HttpPosteDFile file = files[i];                                        //得到文件对象
                if (file.ContentLength > 0)
                {
                    string fileName = file.FileName;
                    string extension = Path.GetExtension(fileName);
                    int num = rnd.Next(5000, 10000);                            //文件名称
                    path = "../../UserFiles/temp/" + num.ToString() + extension;
                    file.SaveAs(System.Web.HttpContext.current.Server.MapPath(path));        //保存文件。
                }
            }
            context.Response.Write(path);            //返回文件存储后的路径，用于回显。
        }
    }

    <img id="server_img" width="360px" style="border: 1px solid #ccc; padding: 2px;"   title="" alt="" />   //用于回显图片
    <asp:FileUpload ID="Up_load" runat="server" onchange="upload()"  ontextchange="upload()"/>        //上传图片，自动的，两个事件是为了保证所有浏览器都兼容。