页面：
<asp:FileUpload ID="FileUpload" runat="server" />
<asp:Button ID="BtnUp" runat="server" onclick="BtnUp_Click" Text="上 传" />
<asp:Label ID="LabMsg" runat="server"></asp:Label>

后台：
/// <summary>
        /// 上传单一图片——有“选择”+“上传”两个按钮
        /// </summary>
        /// <param name="sender"></param>
        /// <param name="e"></param>
        protected void BtnUp_Click(object sender, EventArgs e)
        {
            if (FileUpload.HasFile)
            {
                string savePath = Server.MapPath("~/upload/");//指定上传文件在服务器上的保存路径
                //检查服务器上是否存在这个物理路径，如果不存在则创建
                if (!System.IO.Directory.Exists(savePath))
                {
                    System.IO.Directory.CreateDirectory(savePath);
                }
                savePath = savePath + "\\" + FileUpload.FileName;
                FileUpload.SaveAs(savePath);
                LabMsg.Text = string.Format("<a href='upload/{0}'>upload/{0}</a>", FileUpload.FileName);
                this.img.Src = "upload/" + FileUpload.FileName;
            }
            else
            {
                LabMsg.Text = "你还没有选择上传文件!";
            }
        }