说明：本文实现了
图片格式随意转换（下拉框选择）；
点击FileUpload立即显示图片（Js实现）的技巧；

第一步：打开页面

第二步：选择一副Jpg格式的图片

第三步：转换为GIF格式，明显看出图片画质降低。

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")
{
ConvertImage(FileUpload1.PostedFile.FileName, System.Drawing.Imaging.ImageFormat.Jpeg, serverpath + ".jpg");
}
else if(DropDownList1.SelectedValue == "Bmp")
{
ConvertImage(FileUpload1.PostedFile.FileName, System.Drawing.Imaging.ImageFormat.Bmp, serverpath + ".bmp");
}
else
{
//清清月儿留给大家http://blog.csdn.net/21aspnet
}
}

public void ConvertImage(string Filename, System.Drawing.Imaging.ImageFormat DesiredFormat, string NewFilename)
{
try
{
System.Drawing.Image imgFile = System.Drawing.Image.FromFile(Filename);
imgFile.Save(NewFilename, DesiredFormat);
Image1.ImageUrl = NewFilename;
Label1.Text = "转换成功，生成"+NewFilename+"，如下所示。";
TextBox1.Text = "1";//开始为0，转换后为1
}
catch (Exception ex)
{
Response.Write(ex);
}
}

}

前台代码：
<%@ Page Language="C#" AutoEventWireup="true" CodeFile="Default.aspx.cs" Inherits="_Default" %>

来源：http://blog.csdn.net/21aspnet/