Returns previous image for same content_object and None if image
        is the first.

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