Jag skapar en bild dynamisk som skall ha en transparant bakgrund,men den blir svart iaf varför det?

Response.ContentType = "image/GIF";

        // Create a Bitmap instance that's 468x60, and a
        Graphics instance;

        const int width = 300, height = 300;
        
        Bitmap objBitmap = new Bitmap(width, height);
        
        Graphics objGraphics = Graphics.FromImage(objBitmap);
        // Create a black background for the border
        objGraphics.FillRectangle(new SolidBrush(Color.Transparent), 0, 0,
        width, height);


        // Save the image to a file
        Color backColor = objBitmap.GetPixel(1, 1);
        objBitmap.MakeTransparent(backColor);
        objBitmap.Save(Response.OutputStream, ImageFormat.Gif);
        
        // clean up...
        objGraphics.Dispose();
         objBitmap.Dispose()