c#图片格式装换代码

后台代码：

using System;
using System.Data;
using System.Configuration;
using System.Collections;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;
using System.IO;
using System.Drawing;
public partial class _Default : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
    }
    protected void Button1_Click(object sender, EventArgs e)
    {
        string filepath = FileUpload1.PostedFile.FileName;
        string filename = filepath.Substring(filepath.LastIndexOf("\\") + 1);
        string serverpath = Server.MapPath("images/") + System.DateTime.Now.ToString("yyy-MM-dd-hh-mm-ss") + Session.SessionID + filename;

        if (DropDownList1.SelectedValue == "GIF")
        {
            ConvertImage(FileUpload1.PostedFile.FileName, System.Drawing.Imaging.ImageFormat.Gif, serverpath+".gif");
        }
        else if(DropDownList1.SelectedValue == "Jpeg")