GDI+ Buffered Image and redraw problem

// reverse colors 
  private void button3_Click(object sender, EventArgs e)
  {
      Graphics g = pan.CreateGraphics();
      string fname = "c:\\flag.gif";
      Bitmap bmp = new Bitmap(fname);        
      Bitmap bmpMem = new Bitmap(bmp.Width,bmp.Height);
      Graphics gMem = Graphics.FromImage(bmpMem);
      gMem.DrawImage(bmp,0,0); // Draw flag to memory
      Color cnRed = Color.FromArgb(255,214,41,33);
      for (int y=0; y 240) bmpMem.SetPixel(x,y, cnRed);
            else 
            bmpMem.SetPixel(x,y,Color.White);                 
         }   
      }   
      g.DrawImage(bmpMem,0,0); // Display reversed flag on panel     
      gMem.Dispose();
      g.Dispose();   
  }

// draw image 
   void button1_Click(object sender, EventArgs e)
  {
      Graphics g = pan.CreateGraphics();
      string fname = "c:\\flag.gif";
      Bitmap bmp = new Bitmap(fname);   
      g.DrawImage(bmp,0,0);      
      g.Dispose();      
  }