private Bitmap rotateImage(Bitmap b, float angle)
    //create a new empty bitmap to hold rotated image
    Bitmap returnBitmap = new Bitmap(b.Width, b.Height);
    //make a graphics object from the empty bitmap
    Graphics g = Graphics.FromImage(returnBitmap);
    //move rotation point to center of image
    g.TranslateTransform((float)b.Width / 2, (float)b.Height / 2);
    //move image back
    g.TranslateTransform(-(float)b.Width / 2, -(float)b.Height / 2);
    //draw passed in image onto graphics object
    g.DrawImage(b, new Point(0, 0));
    return returnBitmap;

نحوه استفاده:

private void button1_Click(object sender, EventArgs e)
    Bitmap b = (Bitmap) pictureBox1.Image;
    pictureBox2.Image = rotateImage(b, 60);