要使用asp.net动态输出透明gif图片，也就是用Response.ContentType = "image/GIF"。
查了国内几个中文资料都没解决，最后是在一个英文博客上找到一个可以用的办法。
http://www.codedblog.com/2007/08/28/generating-a-transparent-gif-image-using-c/
他的解决代码是：
代码
//存成gif.ashx
<%@ WebHandler Language="C#" Class="Gif" %>
using System.IO;
using System.Web;
using System.Drawing;
public class Gif : IHttpHandler {
/// <summary>
/// Returns a transparent background GIF image from the specified Bitmap.
/// </summary>
/// <param name="bitmap">The Bitmap to make transparent.</param>
/// <param name="color">The Color to make transparent.</param>
/// <returns>New Bitmap containing a transparent background gif.</returns>
public Bitmap MakeTransparentGif(Bitmap bitmap, Color color) {
byte R = color.R;
byte G = color.G;
byte B = color.B;
MemoryStream fin = new MemoryStream();
bitmap.Save(fin, System.Drawing.Imaging.ImageFormat.Gif);
MemoryStream fout = new MemoryStream((int)fin.Length);
int count = 0;
byte[] buf = new byte[256];
byte transparentIdx = 0;
fin.Seek(0, SeekOrigin.Begin);
//header
count = fin.Read(buf, 0, 13);
if ((buf[0] != 71) || (buf[1] != 73) || (buf[2] != 70)) return null; //GIF
fout.Write(buf, 0, 13);
int i = 0;
if ((buf[10] & 0x80) > 0) {
i = 1 << ((buf[10] & 7) + 1) == 256 ? 256 : 0;
}
for (; i != 0; i--) {
fin.Read(buf, 0, 3);
if ((buf[0] == R) && (buf[1] == G) && (buf[2] == B)) {
transparentIdx = (byte)(256 - i);
}
fout.Write(buf, 0, 3);
}
bool gcePresent = false;
while (true) {
fin.Read(buf, 0, 1);
fout.Write(buf, 0, 1);
if (buf[0] != 0x21) break;
fin.Read(buf, 0, 1);
fout.Write(buf, 0, 1);
gcePresent = (buf[0] == 0xf9);
while (true) {
fin.Read(buf, 0, 1);
fout.Write(buf, 0, 1);
if (buf[0] == 0) break;
count = buf[0];