Returns next image for same content_object and None if image is
        the last.

def next(self):
        ''' Returns next image for same content_object and None if image is
        the last. '''
        try:
            return self.__class__.objects.for_model(self.content_object,
                                                    self.content_type).\
                            filter(order__lt=self.order).order_by('-order')[0]
        except IndexError:
            return None