そもそも直接書くってのが気持ち悪い

        private void Form1_Shown(object sender, EventArgs e)
        {
            Bitmap bmp = new Bitmap(400, 400, PixelFormat.Format32bppArgb);
            using (Graphics g = Graphics.FromImage(bmp))
            {
                g.FillEllipse(Brushes.Red, 50, 50, 100, 100);
                g.Dispose();
            }
            pictureBox1.Image = bmp;
        }