insert and remove marks on picturebox

[rock0]

I'm new to C#, I want insert or remove black X into position in picturebox.
I don't know how can do this.

thanks

my code is here

 private void pictureBox1_MouseClick(object sender, MouseEventArgs e)
        {
            int x = e.X;
            int y = e.Y;
           if ((x >= 1 && x <= 20) && (y >= 1 && y <= 20))
           {
               // I want insert black X here x = 10, y = 10
               // if X exists remove it
           }
           else if ((x >= 25 && x <= 45) && (y >= 1 && y <= 20))
           {
               // I want insert black X here x = 35, y = 10
               //  ...etc
           }
        }

[tinstaafl]

One possible way, could be to have another picturebox loaded with a picture of an 'x', then just show it at the coordinates you want.