2 dependent retrofit requests made by RxJava

Question

I'm using the foursquare API to retrieve photos of nearby venues and display them. I'm doing it by RxJava and Retrofit. The first time I need to request a list of venues with proper ids and the second request should be with ids as parameters to retrieve photos for every single venue. I wanna do everything with single RxJava call because that's the best practice, isn't it? Below is my code, unfortunately, I stuck at the place when I should do the second request. Any hint how should I solve it?

        Observable<VenuesResponse> observable = mApiService.getVenues();

    observable
            .map(new Function<VenuesResponse, List<VenuesItem>>() {
                @Override
                public List<VenuesItem> apply(VenuesResponse venuesResponse) throws Exception {
                    return venuesResponse.getResponse().getVenues();
                }
            })
            .flatMap(new Function<List<VenuesItem>, ObservableSource<VenuesItem>>() {
                @Override
                public ObservableSource<VenuesItem> apply(List<VenuesItem> venuesItems) throws Exception {
                    return Observable.fromIterable(venuesItems);
                }
            })

            //TODO code that requests for photo and adds url to VenuesItem

            .subscribeOn(Schedulers.newThread())
            .observeOn(AndroidSchedulers.mainThread())
            .subscribe();
}