private void Form1_Paint(object sender, PaintEventArgs e)
        {
            e.Graphics.DrawEllipse(Pens.Red, new Rectangle(0, 0, 100, 100));
            e.Graphics.DrawLine(Pens.Red, new Point(0, 0), new Point(100, 100));
        }

------解决方案--------------------
在picture的Paint事件中写一楼的代码。完整的如下:
  private void picture1_Paint(object sender, PaintEventArgs e)
       {
           e.Graphics.DrawEllipse(Pens.Red, new Rectangle(0, 0, 100, 100));
           e.Graphics.DrawLine(Pens.Red, new Point(0, 0), new Point(100, 100));
       }