How to tilt rectangle to 45 degree in windowsform

private void Form1_Load(object sender, EventArgs e)
 {
 Image img = pictureBox1.Image;       
 RotateImage(img, 45);
 }

     public static Image RotateImage(Image img, float rotationAngle)
        {
            //create an empty Bitmap image
            Bitmap bmp = new Bitmap(img.Width, img.Height);

            //turn the Bitmap into a Graphics object
            Graphics gfx = Graphics.FromImage(img);

            //gfx.Clear(Color.Transparent);

           
            //now we set the rotation point to the center of our image
            gfx.TranslateTransform((float)img.Width / 2, (float)img.Height / 2);

            //now rotate the image
            gfx.RotateTransform(rotationAngle);

            gfx.TranslateTransform(-(float)img.Width / 2, -(float)img.Height / 2);

            //set the InterpolationMode to HighQualityBicubic so to ensure a high
            //quality image once it is transformed to the specified size
            gfx.InterpolationMode = InterpolationMode.HighQualityBicubic;
            //gfx.Clear(Color.White);
            //now draw our new image onto the graphics object
            gfx.DrawImage(img, new Point(0, 0));

            //dispose of our Graphics object
            gfx.Dispose();

            //return the image
            return img;
        
    }