Bitmap b = new Bitmap(pictureBox2.Image);
        b.MakeTransparent(b.GetPixel(1, 1));
        pictureBox2.Image = b;

PictureBox2.BackColor = Color.Black;
Bitmap bmp = new Bitmap (pictureBox2.Image);
bmp.MakeTransparent(Color.Black);
pictureBox2.Image = bmp;