Bild rotieren mit C#

public Bitmap rotateImage(Bitmap bitmap, float angle)
        {
            using (Graphics graphics = Graphics.FromImage(bitmap))
            {
                graphics.TranslateTransform((float)bitmap.Width / 2, (float)bitmap.Height / 2);
                graphics.RotateTransform(angle);
                graphics.TranslateTransform(-(float)bitmap.Width / 2, -(float)bitmap.Height / 2);
                graphics.DrawImage(bitmap, new Point(0, 0));
            }
            return bitmap;
        }

Bitmap btmp1 = new Bitmap();
btmp1.RotateFlip(RotateFlipType.Rotate180FlipX);

Bitmap returnBitmap = new Bitmap(bitmap.Width, bitmap.Height);