private void dataGridView1_CellClick(object sender,
    DataGridViewCellEventArgs e)
{

    if (turn.Text.Equals(gameOverString)) { return; }

    DataGridViewImageCell cell = (DataGridViewImageCell)
        dataGridView1.Rows[e.RowIndex].Cells[e.ColumnIndex];

    if (cell.Value == blank)
    {
        if (IsOsTurn())
        {
            cell.Value = o;
        }
        else
        {
            cell.Value = x;
        }
        ToggleTurn();
    }
    if (IsAWin())
    {
        turn.Text = gameOverString;
    }
}