private void NowPaint1()
  {
    Rectangle rec = new Rectangle(0, 0, panel1.Width, panel1.Height);
    GraphicsPath round = CreateRound(rec, 10 * 2);
    Graphics gp = panel1.CreateGraphics();
    Brush b = new LinearGradientBrush(rec, Color.Blue, Color.Red, LinearGradientMode.Vertical); 
    gp.FillPath(b, round);
    Image im = panel1.BackgroundImage; if (im != null) gp.DrawImage(im, new Point(0, 0));
  }

  private void NowPaint2()
  {
   //在这里如何清空已经绘制的图
    panel1.BackColor = Color.Transparent;
  }


  private GraphicsPath CreateRound(Rectangle rect, int radius) //画圆角
  {
    GraphicsPath roundRect = new GraphicsPath();
    roundRect.AddArc(rect.Right - radius, rect.Top, radius, radius, 270, 90);//右上角

    roundRect.AddArc(rect.Right - 1 - radius, rect.Bottom - 1 - radius, radius, radius, 0, 90);//右下角

    roundRect.AddArc(rect.Left - 1, rect.Bottom - 1 - radius, radius, radius, 90, 90);//左下角

    roundRect.AddArc(rect.Left, rect.Top, radius, radius, 180, 90);//左上角

    return roundRect;
  }