用C#做绘图软件时怎样保存PictureBox中用graphics画的图（不是导入的图片，是自己绘制的）

        public Form1()
        {
            InitializeComponent();
            pictureBox1.Image = new Bitmap(pictureBox1.Width, pictureBox1.Height);
            SolidBrush b = new SolidBrush(Color.White);
            Graphics.FromImage(pictureBox1.Image).FillRectangle(b, 0, 0, pictureBox1.Width, pictureBox1.Height);
        }

        private void pictureBox1_MouseMove(object sender, MouseEventArgs e)
        {
            Graphics.FromImage(pictureBox1.Image).DrawEllipse(new Pen(Brushes.Red), e.X, e.Y, 1, 1);
            pictureBox1.Invalidate();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            pictureBox1.Image.Save(@"D:\1.jpg", ImageFormat.Jpeg);
            MessageBox.Show("ok");
        }