Example of prefetching images with evernotes JobScheduler https://github.com/evernote/android-job and Picasso http://square.github.io/picasso/

build.gradle

dependencies {
    compile 'com.evernote:android-job:1.0.2'
    compile 'com.squareup.picasso:picasso:2.5.2'
}

PrefetchImageJob.java

package de.pfaffenrodt.job;

/**
 * Copyright (C) 2016 Dimitri Pfaffenrodt
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

import android.support.annotation.NonNull;
import android.util.Log;

import com.evernote.android.job.Job;
import com.evernote.android.job.JobRequest;
import com.evernote.android.job.util.support.PersistableBundleCompat;
import com.squareup.picasso.Callback;
import com.squareup.picasso.MemoryPolicy;
import com.squareup.picasso.Picasso;

import net.vrallev.android.cat.Cat;

import java.util.concurrent.CountDownLatch;

public class PrefetchImageJob extends Job {

    public static final String TAG = "PrefetchImageJob";
    public static final String EXTRAS_IMAGES = "images";


    /**
     * @param images
     * @return The unique ID for this job.
     */
    public static int createJob(String[] images){
        PersistableBundleCompat extras = new PersistableBundleCompat();
        extras.putStringArray(PrefetchImageJob.EXTRAS_IMAGES, images);

       return new JobRequest.Builder(PrefetchImageJob.TAG)
               .setExecutionWindow(30_000L, 40_000L)
               .setRequiredNetworkType(JobRequest.NetworkType.CONNECTED)
               .setExtras(extras)
               .build()
               .schedule();
    }

    @NonNull
    @Override
    protected Result onRunJob(Params params) {
        final String[] images = params.getExtras().getStringArray(EXTRAS_IMAGES);
        if(images == null){
            return Result.FAILURE;
        }
        final CountDownLatch latch = new CountDownLatch(images.length);
        for (int i = 0; i < images.length; i++) {
            final String image = images[i];
            if(!isValidImage(image)){
                latch.countDown();
                continue;
            }
            Picasso.with(getContext())
                    .load(image)
                    .memoryPolicy(MemoryPolicy.NO_CACHE)
                    .fetch(new Callback() {
                        @Override
                        public void onSuccess() {
                            Log.d(TAG, "onSuccess: prefetched image:"+image);
                            latch.countDown();
                        }

                        @Override
                        public void onError() {
                            cancel();
                            Log.d(TAG, "onError: not prefetched image:" + image);
                            latch.countDown();
                        }
                    });
        }

        try {
            latch.await();
        } catch (InterruptedException e) {
            Cat.e(e);
        }
        if (isCanceled()) {
            return params.isPeriodic() ? Result.FAILURE : Result.RESCHEDULE;
        } else {
            return Result.SUCCESS;
        }
    }

    private boolean isValidImage(String imageUrl) {
        return imageUrl!=null && !imageUrl.trim().isEmpty();
    }
}

SampleActivity.java

package de.pfaffenrodt.job;

import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;

import com.evernote.android.job.Job;
import com.evernote.android.job.JobCreator;
import com.evernote.android.job.JobManager;
import com.evernote.android.job.JobRequest;
import com.evernote.android.job.util.support.PersistableBundleCompat;

import de.pfaffenrodt.job.PrefetchImageJob;

public class MainActivity extends AppCompatActivity {

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

        JobManager.create(this).addJobCreator(new JobCreator() {
            @Override
            public Job create(String tag) {
                return new PrefetchImageJob();
            }
        });
        PrefetchImageJob.createJob(new String[]{
                "http://lorempixel.com/400/200/",
                "http://lorempixel.com/200/300/",
                "http://lorempixel.com/400/500/"
        });
    }

    @Override
    protected void onDestroy() {
        super.onDestroy();
        JobManager.instance().cancelAll();
    }
}