System.Drawing.Image _gifImage;

        _gifImage = System.Drawing.Image.FromFile(Server.MapPath("white.JPG"));
        Bitmap bm = new Bitmap(_gifImage.Width, _gifImage.Height, 
                PixelFormat.Format8bppIndexed);

        // Get the palette from the bitmap
        ColorPalette pal = bm.Palette;

        // Set Alpha to 0
        for (int i = 0; i < pal.Entries.Length; i++)
        {
            Color col = pal.Entries[i];
            pal.Entries[i] = Color.FromArgb(0, col.R, col.G, col.B);
        }

        // assign palette
        bm.Palette = pal;

        //to copy the bitmap data we need to lock the source & 

    //destination bits
        BitmapData src = ((Bitmap)_gifImage).LockBits(new Rectangle
        (0, 0, _gifImage.Width, _gifImage.Height), ImageLockMode.ReadOnly,
         _gifImage.PixelFormat);
        BitmapData dst = bm.LockBits(new Rectangle(0, 0, bm.Width, bm.Height), 
        ImageLockMode.WriteOnly, bm.PixelFormat);

        //finished, unlock the bits
        ((Bitmap)_gifImage).UnlockBits(src);
        bm.UnlockBits(dst);
        
        //Set Response Type to "image/gif"
        Response.ContentType = "image/gif";

        //Writing the gif directly into the Output-Stream
        bm.Save(Response.OutputStream, ImageFormat.Gif);

        //cleaning up
        bm.Dispose();
        _gifImage.Dispose();

        //Send output stream
        Response.Flush();