如何在ASP.NET页面中自动生成并显示透明的GIF图片

在页面中加入一个图片控件，并将它的Style属性设为：FILTER: chroma(color:#000000) ，原因是自动生成的GIF图片总是黑色背景。然后在后台代码中写入如下代码。(见DEMO)
using System;
using System.Collections;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Web;
using System.Web.SessionState;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.HtmlControls;
using System.IO;
using System.Drawing.Drawing2D;
namespace WebPageIndex
{
/**//// <summary>
/// WebForm1 的摘要说明。
/// </summary>
public class WebForm1 : System.Web.UI.Page
{
protected System.Web.UI.HtmlControls.HtmlImage IMG1;

private void Page_Load(object sender, System.EventArgs e)
{
// 在此处放置用户代码以初始化页面
MyTempImage myTempImage=new MyTempImage();
IMG1.Src=myTempImage.CreateImage();

}
public class MyTempImage : Page
{
public string CreateImage()
{
string str="兴中公司财务章";
Bitmap image=new Bitmap(300,300);
Graphics g=Graphics.fromImage(image);
string thefullname=Server.MapPath("/")+"\\nowtime.gif";
Pen p=new Pen(Brushes.Red,10);
Pen p1=new Pen(Brushes.Red,5);
SolidBrush b=new SolidBrush(Color.Red);
g.SmoothingMode=SmoothingMode.AntiAlias;
Point point1 = new Point( 150, 50);
Point point2 = new Point(50, 150);
Point point3 = new Point(230, 150);

Point[] curvePoints =
{
point1,
point2,
point3,
};
g.FillPolygon(b,curvePoints,FillMode.Winding );
g.DrawEllipse(p,10,10,280,280);
g.DrawString(str,new Font("隶书",20,FontStyle.Bold),new
SolidBrush(Color.fromArgb(255, 255,0