private void checkBoxAchtergrond_CheckedChanged(object sender, EventArgs e)
        {
            if (checkBoxAchtergrond.Checked == true)
            {
                this.BackgroundImage = Properties.Resources.achtergrondH11Oef4;
                this.BackgroundImageLayout = ImageLayout.Stretch;
            }
            else
            {
                this.BackgroundImage = null;
            }
        }