c# windows forms draw pixel

private void SetPixel_Example(PaintEventArgs e)
{

    // Create a Bitmap object from a file.
    Bitmap myBitmap = new Bitmap("Grapes.jpg");

    // Draw myBitmap to the screen.
    e.Graphics.DrawImage(myBitmap, 0, 0, myBitmap.Width,
        myBitmap.Height);

    // Set each pixel in myBitmap to black.
    for (int Xcount = 0; Xcount < myBitmap.Width; Xcount++)
    {
        for (int Ycount = 0; Ycount < myBitmap.Height; Ycount++)
        {
            myBitmap.SetPixel(Xcount, Ycount, Color.Black);
        }
    }

    // Draw myBitmap to the screen again.
    e.Graphics.DrawImage(myBitmap, myBitmap.Width, 0,
        myBitmap.Width, myBitmap.Height);
}