Source (adapted for GenericImage) -> http://stackoverflow.com/a/11176961/341706

/// <summary>
        /// Rotates the image.
        /// </summary>
        /// <param name="image">
        /// The source image to rotate.
        /// </param>
        /// <param name="angle">
        /// The angle in degrees to rotate the image.
        /// </param>
        /// <param name="centerX">The x position of the rotation axis.</param>
        /// <param name="centerY">The y position of the rotation axis.</param>
        /// <returns>
        /// Returns a new rotated image.
        /// </returns>
        public static GenericImage<T> Rotate<T>(this GenericImage<T> image, int centerX, int centerY, float angle)
        {
            // source (adapted for GenericImage) -> http://stackoverflow.com/a/11176961/341706
            var radians = (Math.PI / 180) * angle;
            var cos = Math.Cos(radians);
            var sin = Math.Sin(radians);
            var newImage = new GenericImage<T>(image.Width, image.Height);

            for (var x = 0; x < image.Width; x++)
                for (var y = 0; y < image.Height; y++)
                {
                    var m = x - centerX;
                    var n = y - centerY;
                    var j = ((int)(m * cos + n * sin)) + centerX;
                    var k = ((int)(n * cos - m * sin)) + centerY;
                    if (j >= 0 && j < image.Width && k >= 0 && k < image.Height)
                    {
                        newImage[x, y] = image[j, k];
                    }
                }

            return newImage;
        }