[WebMethod(EnableSession = true)]
public void Draw(Point[] points)
{
    Image scribbleImage = (Image)Session["Image"];
    lock(scribbleImage)
    {
        using (Graphics g = Graphics.FromImage(scribbleImage))
        using(Pen p = new Pen(Color.Black, 2))
        {
            if (points.Length > 1)
            {
                int startX = points[0].X;
                int startY = points[0].Y;

                for (long i = 1; i < points.Length; i++)
                {
                    g.DrawLine(p, startX, startY,
                        points[i].X, points[i].Y);
                    startX = points[i].X;
                    startY = points[i].Y;
                }
            }
        }
    }
}