gistfile1.cs

public class MainPageViewModel : ReactiveObject
{
    ObservableAsPropertyHelper<BitmapImage> _OriginalImage;
    public BitmapImage OriginalImage {
        get { return _OriginalImage.Value; }
    }

    public ReactiveCommand TakePhoto { get; protected set; }

    public MainPageViewModel()
    {
        TakePhoto = new ReactiveCommand();

        var cameraUI = new CameraCaptureUI();

        TakePhoto.SelectAsync(async x => await cameraUI.CaptureFileAsync(CameraCaptureUIMode.Photo))
            .ObserveOn(RxApp.DeferredScheduler)
            .Select(x => new BitmapImage(new Uri(x.Path)))
            .ToProperty(this, x => x.OriginalImage);
    }
}