Graphics g = pictureBox1.CreateGraphics();
Pen a = new Pen(Color.Red, 6);
g.DrawRectangle(a, 100, 100, 50, 100);
Bitmap bm = new Bitmap(pictureBox1.Width, pictureBox1.Height);
pictureBox1.DrawToBitmap(bm, new Rectangle(0, 0, bm.Width, bm.Height));
SaveFileDialog sfd = new SaveFileDialog();
if (sfd.ShowDialog() == DialogResult.OK)
{
bm.Save(sfd.FileName);
}