Image.Source = await CropImageAsync(Image.Source, new Rectangle(10, 10, 100, 100));

CropUwpImageControl.cs

private async Task<SoftwareBitmapSource> CropImageAsync(ImageSource source, Rectangle rectangle)
{
    try
    {
        SoftwareBitmap bitmap;

        var image = source as BitmapImage;
        var file = await StorageFile.GetFileFromApplicationUriAsync(image.UriSource);
        using (var stream = await file.OpenAsync(FileAccessMode.Read))
        {
            var decoder = await BitmapDecoder.CreateAsync(stream);
            bitmap = await decoder.GetSoftwareBitmapAsync(BitmapPixelFormat.Bgra8, BitmapAlphaMode.Premultiplied);
        }

        using (var memory = new InMemoryRandomAccessStream())
        {
            var encoder = await BitmapEncoder.CreateAsync(BitmapEncoder.BmpEncoderId, memory);
            encoder.SetSoftwareBitmap(bitmap);
            encoder.BitmapTransform.Bounds = new BitmapBounds()
            {
                X = (uint)rectangle.X,
                Y = (uint)rectangle.Y,
                Height = (uint)rectangle.Height,
                Width = (uint)rectangle.Width
            };
            await encoder.FlushAsync();
            var decoder = await BitmapDecoder.CreateAsync(memory);
            bitmap = await decoder.GetSoftwareBitmapAsync(bitmap.BitmapPixelFormat, bitmap.BitmapAlphaMode);
        }

        if (bitmap.BitmapPixelFormat != BitmapPixelFormat.Bgra8 || bitmap.BitmapAlphaMode == BitmapAlphaMode.Straight)
        {
            bitmap = SoftwareBitmap.Convert(bitmap, BitmapPixelFormat.Bgra8, BitmapAlphaMode.Premultiplied);
        }

        var output = new SoftwareBitmapSource();
        await output.SetBitmapAsync(bitmap);
        return output;
    }
    catch (Exception)
    {
        throw;
    }
}