In mousedown event after (button.DoDragDrop....statement) I checked if the button backcolor is red then I assigned the button image to null else I assigned the knight image to the button image.
here is the codes :
private void btn_MouseDown(object sender, MouseEventArgs e)
{
Button button = sender as Button;
if (e.Button == MouseButtons.Left && button.Image != null)
{
button.DoDragDrop(button.Image, DragDropEffects.Move);
if (button.BackColor == Color.Red)
button.Image = null;
else
button.Image = pictureBox1.Image;
}
}
And eveything is ok now :)