private void locatePointButton_Click(object sender, EventArgs e)
{
pictureBox.Capture = true;
Button sourceButton = (Button)sender;
pointIndexToLocate = int.Parse((string)sourceButton.Tag);
pointPreviousValue = imagePoints[pointIndexToLocate];
imagePoints[pointIndexToLocate] = emptyPoint;
statusLabel.Text = string.Format("Locate point #{0} in the image ...", pointIndexToLocate + 1);
pictureBox.Invalidate();
}