上传图片生成缩略图

asp:FileUpload ID="FileUpload1" runat="server" />
<asp:Button ID="btnaddd" runat="server" Text="上传" OnClick="btnaddd_Click" />
<div id="zyc"></div>

<asp:Label ID="lb" runat="server" Text="" ForeColor="red"></asp:Label>
function AddFileUpLoad(Str)
{
var oDiv=document.getElementById("zyc");

oDiv.style.visibility="visible";
oDiv.innerHTML="<img src=\""+document.getElementById(Str).value+"\">";

// oDiv.innerHTML=document.getElementById(Str).value;;
}

</script>
cs代码：

protected void Page_Load(object sender, EventArgs e)
    {
       if(!IsPostBack)
       {
           this.FileUpload1.Attributes.Add("onchange", "AddFileUpLoad('" + this.FileUpload1.ClientID + "')");
       }

        //txttime.Text = DateTime.Now.ToString("yyyy-MM-dd");
    }
   
    protected void btnaddd_Click(object sender, EventArgs e)
    {

               //图片上传

        if (FileUpload1.PostedFile.ContentLength > 1024 * 1024 * 2)
        {
            lb.Text = "上传的图片不能超过 2 M";
            return;
        }
        string filepath = "";
        string filename = "";

        if (FileUpload1.PostedFile.ContentLength != 0)
        {
            filepath = Server.MapPath("../upfile/" + DateTime.Now.ToString("yyyyMMddHHmmss") + Path.GetFileName(FileUpload1.PostedFile.FileName));
            if (FileUpload1.PostedFile.ContentType.Equals("image/gif") || FileUpload1.PostedFile.ContentType.Equals("image/pjpeg") || FileUpload1.PostedFile.ContentType.Equals("image/JPG"))
            {
                if (FileUpload1.PostedFile != null)
                    try
                    {
                        FileUpload1.PostedFile.SaveAs(filepath);
                    }
                    catch (Exception exc)
                    {
                        lb.Text = "保存文件时出错<b>" + filepath + "</b><br>" + exc.ToString();
                        return;
                    }
                filename = "upfile/" + DateTime.Now.ToString("yyyyMMddHHmmss") + Path.GetFileName(FileUpload1.PostedFile.FileName);
              
              

              
            }
            else
            {
                lb.Text = "请上传JPG格式、GIF格式或JPG格式的图片！";
                return;
            }
        }
        else {
          lb.Text = "请上传图片！";
            return;
        }

              }