How to get data from a method in Android in MainActivity?

Question

Java

Good day, dear community. I'm trying to implement one psychedelic idea (outputting random images on the screen), I'm facing this problem. When a static method is called in MainActivity, the application crashes. I feel like this is a really stupid mistake. Please tell me how to do it right and why? Thank you very much in advance for your reply.
Here is the code.

package com.eningapps.flickrrandomview;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.ImageView;
import com.flickr4java.flickr.Flickr;
import com.flickr4java.flickr.FlickrException;
import com.flickr4java.flickr.REST;
import com.flickr4java.flickr.photos.Photo;
import com.flickr4java.flickr.photos.PhotoList;
import com.flickr4java.flickr.photos.SearchParameters;
import com.squareup.picasso.Picasso;

public class MainActivity extends AppCompatActivity {

    public static String getUrl() {
        String[] SEARCHWORD={"dont worry"};

        String apikey = "ТУТМОЙПАРОЛЬ";
        String secret = "ТУТМОЙПАРОЛЬ";

        Flickr flickr = new Flickr(apikey, secret, new REST());
        SearchParameters searchParameters = new SearchParameters();
        searchParameters.setAccuracy(1);
        searchParameters.setTags(SEARCHWORD);

        String rez="";

        for (int i = 0; true; i++) {
            try {
                PhotoList<Photo> list = flickr.getPhotosInterface().search(searchParameters, 10, i); 
//Глохнет конкретно в строке выше.Потому что идёт запрос на сервер, а я его выполняю в основном потоке?
                if (list.isEmpty())
                    break;
                for(Photo ph: list){
                    rez=ph.getLargeUrl();
                    if(!rez.equals("")) break;
                }
            } catch (FlickrException e) {
                e.printStackTrace();
            }
        }
        return rez;
    }

    ImageView iV;
    Button button;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        iV=(ImageView) findViewById(R.id.imageView);
        button=(Button) findViewById(R.id.button);
        button.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                Picasso.with(getApplicationContext()) 
                        .load(getUrl())
                        .resize(300, 300) 
                        .into(iV); // а
            }
        });
    }
}