Hi +Ali Rahimpour
If you're not using our movie data methods then you'll mostly be on your own to build whatever workflow you need. The assumption all throughout the API is that you would be using our service for the data itself and if that was the case, you would find that every movie info call:https://api.themoviedb.org/3/movie/131631?api_key=###&language=sv
Has both 'poster_path' and 'backdrop_path' fields which is all you need. For your case, you wouldn't need any extra API calls at all. These 2 fields are available on every single object (search results, featured list items, movie details, etc…)
If you're not using our data methods, how ever you're matching our IDs to their IDs, you'll then have to head off and get the images via the /images API method or just the regular movie method (linked above).
Whether you use the call I linked to above ^ or are using search results, like I said, these fields are available in all of these calls so the most you need to do is 1 API call to TMDb.
I will add that, most people in this case would create their own local web service that merges and caches the 2 web services together so that you can call a single source that is perfectly mapped to your schema in your Android app.
Hope that helps.