He estado haciendo una aplicación de película (proyecto de trabajo para aprender Android en udacity por cierto). Encontré algunos problemas, pero los resolví siguiendo hilos como estosNo se puede modificar ArrayAdapter en ListView: UnsupportedOperationException

Sin embargo, aunque mi aplicación no se bloquea, las imágenes no se cargan y no puedo entender por qué. Aquí está mi código:

Diseño de archivos XMl movies_item.xml

<?xml version="1.0" encoding="utf-8"?>
<ImageView xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:id = "@+id/movies_item_thumbnail"
    >
</ImageView>

fragment_main.xml

<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools" android:layout_width="match_parent"
    android:layout_height="match_parent" android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    android:paddingBottom="@dimen/activity_vertical_margin"
    tools:context="com.example.cindy.popularmovies.MoviesFragment">

    <GridView
        android:id="@+id/movies_gridview"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:numColumns="auto_fit"
        android:gravity="center"
        />

</FrameLayout>

MovieArrayAdapter.java

    public class MovieArrayAdapter extends ArrayAdapter<Movie> {
        private final Context context;
        private final List<Movie> movies;

        public MovieArrayAdapter(Context context,List<Movie> movies) {
            super(context,0,movies);
            this.context = context;
            this.movies = movies;
        }

        /*
        * Within the getView() method you would inflate an XML based layout and
        * then set the content of the individual views based on the Java object for this row.
        * */

        @Override
        public View getView(int position, View convertView, ViewGroup parent) {

            View rowView = convertView;
            if(rowView == null) {
                //Inflate the XML based Layout
                LayoutInflater inflater = (LayoutInflater) context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
                rowView = inflater.inflate(R.layout.movies_item, parent, false);

                //Get the ImageView
                ImageView movieThumbnail = (ImageView) rowView.findViewById(R.id.movies_item_thumbnail);


                //Load Image into the ImageView
                Picasso.with(context).load(movies.get(position).getThumbnail()).into(movieThumbnail);

                Log.v("Populating", movies.get(position).getThumbnail());
            }
            return rowView;
        }

        @Override
        public int getCount() {
            return super.getCount();
        }

        @Override
        public int getPosition(Movie item) {
            return super.getPosition(item);
        }
    }


MovieFragment.java

public class MoviesFragment extends Fragment {

    private MovieArrayAdapter mMoviesAdapter;

    public MoviesFragment() {
    }

    @Override
    public void onStart() {
        super.onStart();
        new FetchMoviesTask().execute();
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        View rootView = inflater.inflate(R.layout.fragment_main, container, false);

        mMoviesAdapter = new MovieArrayAdapter(getActivity(),new ArrayList<Movie>());

        GridView moviesGridView = (GridView) rootView.findViewById(R.id.movies_gridview);
        moviesGridView.setAdapter(mMoviesAdapter);

        return rootView;
    }

    public class FetchMoviesTask extends AsyncTask<String,Void,Movie[]> {

        final String LOG_TAG = this.getClass().getSimpleName();

        @Override
        protected Movie[] doInBackground(String...params) {

            //Retrieve the Popular Movies Json String
            String moviesJsonString = getMoviesJsonString();

            //Parse the Json String to get important data
            try {
                return getPopularMoviesInfo(moviesJsonString);
            } catch(JSONException e) {
                Log.e(LOG_TAG,e.getMessage(),e);
            }
            return null;
        }

        @Override
        protected void onPostExecute(Movie[] movies) {
            if(movies != null) {
                mMoviesAdapter.clear();
                mMoviesAdapter.addAll(movies);
            }
        }

        /*
                * This Function is to retrieve the Popular movies Json String from a WEB API
                * */
        private String getMoviesJsonString() {
            HttpURLConnection urlConnection = null;
            BufferedReader bufferedReader = null;
            String moviesJsonString = null;

            try {

                //Set up the URI
                final String MOVIES_BASE_URL = "http://api.themoviedb.org/3/discover/movie?";
                final String SORT_PARAM = "sort_by";
                final String API_PARAM ="api_key";

                Uri moviesUri = Uri.parse(MOVIES_BASE_URL).buildUpon()
                        .appendQueryParameter(SORT_PARAM, "popularity.desc")
                        .appendQueryParameter(API_PARAM,getString(R.string.movies_api))
                        .build();

                //Open the connection
                URL url = new URL(moviesUri.toString());
                urlConnection = (HttpURLConnection)url.openConnection();
                urlConnection.setRequestMethod("GET");
                urlConnection.connect();

                //Read the input stream into a string
                InputStream inputStream = urlConnection.getInputStream();
                if(inputStream == null) {
                    return null;
                }

                bufferedReader = new BufferedReader(new InputStreamReader(inputStream));

                String line;
                StringBuffer buffer = new StringBuffer();
                while((line = bufferedReader.readLine()) != null) {
                    buffer.append(line + "\n");
                }

                if(buffer.length() == 0){
                    //Stream was empty
                    return null;
                }

                moviesJsonString = buffer.toString();

            }catch(IOException e) {
                Log.e(LOG_TAG,"Error",e);
            } finally {
                if (urlConnection != null) {
                    urlConnection.disconnect();
                }
                if(bufferedReader != null) {
                    try {
                        bufferedReader.close();
                    } catch(IOException e) {
                        Log.e(LOG_TAG, "Error closing stream", e);
                    }
                }
            }

            return moviesJsonString;
        }

        /*
        * This function is to parse the JSON String to retrieve info
        * */
        private Movie[] getPopularMoviesInfo(String jsonString) throws JSONException{

            //Base URL of thumbnail
            final String THUMBNAIL_BASE_URL = " http://image.tmdb.org/t/p/w185";

            //The attribute names we are interested in the JSON String
            final String M_RESULTS = "results";
            final String M_THUMBNAIL = "backdrop_path";
            final String M_TITLE = "original_title";
            final String M_ID = "id";
            final String M_SYPNOSIS = "overview";
            final String M_USER_RATING = "vote_average";
            final String M_RELEASE_DATE = "release_date";


            //Convert Json String to Json Object
            JSONObject moviesJsonObject = new JSONObject(jsonString);

            //Get the Json Array
            JSONArray moviesJsonArray = moviesJsonObject.getJSONArray(M_RESULTS);

            Movie[] movies = new Movie[moviesJsonArray.length()];
            for(int i=0;i<moviesJsonArray.length();i++) {
                JSONObject movie = moviesJsonArray.getJSONObject(i);

                movies[i] = new Movie(THUMBNAIL_BASE_URL + movie.getString(M_THUMBNAIL),
                                    movie.getString(M_TITLE),
                                    movie.getString(M_ID),
                                    movie.getString(M_SYPNOSIS),
                                    movie.getString(M_USER_RATING),
                                    movie.getString(M_RELEASE_DATE));

                Log.v("Movie",movie.getString(M_TITLE));
            }

            return movies;

        }
    }
}

No hay error, pero las imágenes no se están cargando. He estado buscando en otros hilos, pero no puedo encontrar la respuesta a mi problema. Muchas gracias.

[ACTUALIZADO] Cambió el código según la discusión a continuación:

@Override
    public View getView(int position, View convertView, ViewGroup parent) {

        if(convertView == null) {
            //Inflate the XML based Layout
            LayoutInflater inflater = (LayoutInflater) context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
            convertView = inflater.inflate(R.layout.movies_item, parent, false);
        }

        //Get the ImageView
        ImageView movieThumbnail = (ImageView) convertView.findViewById(R.id.movies_item_thumbnail);

        //Load Image into the ImageView
        Picasso.with(context).load(movies.get(position).getThumbnail()).into(movieThumbnail);

        Log.v("Populating", movies.get(position).getThumbnail());

        return convertView;
    }