关于GDI+生成透明背景索引图的问题

Question

最近做一个拼合图片的程序，需求是拼合两张图片，然后把拼合后的图片设置透明。在百度上搜到如下代码，并根据实际需求修改了。最终生成的图片背景始终不透明。因为美工说他把bg1.png图片设置成白色的,然后又是透明的，而查看起来是白色,另外美工说生成索引图才能透明，我初次接触图形开发，一头雾水。大家帮帮我吧，谢谢了。代码如下： 
System.Drawing.Image image = System.Drawing.Image.FromFile(Server.MapPath("bg1.png"));//@"c:\1.jpg"
            System.Drawing.Image sy = System.Drawing.Image.FromFile(Server.MapPath("sz.png"));
            System.Drawing.Image sy2 = System.Drawing.Image.FromFile(Server.MapPath("hard.png"));
            System.Drawing.Image sy3 = System.Drawing.Image.FromFile(Server.MapPath("hard1.png"));
            System.Drawing.Graphics g = System.Drawing.Graphics.FromImage(image);
            g.Clear(Color.White);
            g.DrawString("顶顶顶", new System.Drawing.Font("隶书", 24), System.Drawing.Brushes.Blue, new System.Drawing.PointF(20, 200));
            g.DrawString("再顶定", new System.Drawing.Font("宋体", 18), System.Drawing.Brushes.Black, new System.Drawing.PointF(300, 10));
            //g.DrawImage(sy, new System.Drawing.Point(400, 300));            g.DrawImage(sy, new System.Drawing.Point(10, 10));
            g.DrawImage(sy2, new System.Drawing.Point(15, 15));
            g.DrawImage(sy3, new System.Drawing.Point(20, 20));
            image.Save(Response.OutputStream, System.Drawing.Imaging.ImageFormat.Gif);
            image.Dispose();
            g.Dispose();