public async Task SaveImage()
{

            var grid = (GetView() as ShellView).FindName("img") as Grid;

            RenderTargetBitmap renderTargetBitmap = new RenderTargetBitmap();
            await renderTargetBitmap.RenderAsync(grid, StepInfo.CroppedImage.Width, StepInfo.CroppedImage.Height);

            IBuffer pixels = await renderTargetBitmap.GetPixelsAsync();
            WriteableBitmap wb = new WriteableBitmap(renderTargetBitmap.PixelWidth, renderTargetBitmap.PixelHeight);

            pixels.CopyTo(wb.PixelBuffer);

            //Tutaj trzeba zdobyć obiekt StorageFile

            wb.Save(storageFileInstance) //defaultowo nie ma funkcji Save, wiec podaje kod do tego
}


 public static async Task Save(this WriteableBitmap writeableBitmap, StorageFile outputFile, Guid encoderId)
        {
            Stream stream = writeableBitmap.PixelBuffer.AsStream();
            byte[] pixels = new byte[(uint)stream.Length];
            await stream.ReadAsync(pixels, 0, pixels.Length);

            using (var writeStream = await outputFile.OpenAsync(FileAccessMode.ReadWrite))
            {
                var encoder = await BitmapEncoder.CreateAsync(encoderId, writeStream);
                encoder.SetPixelData(
                    BitmapPixelFormat.Bgra8,
                    BitmapAlphaMode.Premultiplied,
                    (uint)writeableBitmap.PixelWidth,
                    (uint)writeableBitmap.PixelHeight,
                    96,                 // TODO: Shouldnt we get this from the image?
                    96,
                    pixels);
                await encoder.FlushAsync();

                using (var outputStream = writeStream.GetOutputStreamAt(0))
                {
                    await outputStream.FlushAsync();
                }
            }
        }


        public static async Task Save(this WriteableBitmap writeableBitmap, StorageFile outputFile)
        {
            Guid encoderId;

            var ext = "." + outputFile.FileType.ToLower();

            if (new[] { ".bmp", ".dib" }.Contains(ext))
            {
                encoderId = BitmapEncoder.BmpEncoderId;
            }
            else if (new[] { ".tiff", ".tif" }.Contains(ext))
            {
                encoderId = BitmapEncoder.TiffEncoderId;
            }
            else if (new[] { ".gif" }.Contains(ext))
            {
                encoderId = BitmapEncoder.GifEncoderId;
            }
            else if (new[] { ".jpg", ".jpeg", ".jpe", ".jfif", ".jif" }.Contains(ext))
            {
                encoderId = BitmapEncoder.JpegEncoderId;
            }
            else if (new[] { ".hdp", ".jxr", ".wdp" }.Contains(ext))
            {
                encoderId = BitmapEncoder.JpegXREncoderId;
            }
            else
            {
                encoderId = BitmapEncoder.PngEncoderId;
            }

            await writeableBitmap.Save(outputFile, encoderId);
        }