Как я поворачиваю изображение в WinForms

Вот метод, который вы можете использовать для поворота изображения в C #:

/// <summary>
/// method to rotate an image either clockwise or counter-clockwise
/// </summary>
/// <param name="img">the image to be rotated</param>
/// <param name="rotationAngle">the angle (in degrees).
/// NOTE: 
/// Positive values will rotate clockwise
/// negative values will rotate counter-clockwise
/// </param>
/// <returns></returns>
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(bmp);

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

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

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

    //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 bmp;
}