// if the combo box of the gray level is changed, these method will be called
private void comboBox1_SelectedIndexChanged(object sender, EventArgs e)
{
checkPictureBox1();
Bitmap bit = new Bitmap(pictureBox1.Image, pictureBox1.Image.Width, pictureBox1.Image.Height);
pictureBox2.Refresh();
pictureBox2.Image = GraphicClass.changeGrayLevel(Convert.ToInt32(comboBox1.Text), bit);
}