how to send notifications when the app is in the background? Android Studio

I need to make an app for android in which images with information of events are displayed. This I already have what I want to do is that when the app is in the background the Asynctask is run every so often to know if a new image has been added and send a notification.

Here's the code:

    public class MainEventos extends AppCompatActivity {

        private 

ListView listView;
    ArrayList imagen = new ArrayList();
    Bitmap photo;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main_eventos);

        listView = (ListView)findViewById(R.id.listview);
        descargarImagen();
    }
    protected void onStop() {
        super.onStop();

    }
    public void descargarImagen(){
        imagen.clear();
        final ProgressDialog progressDialog = new ProgressDialog(MainEventos.this);
        progressDialog.setMessage("Cargando Imagenes");
        progressDialog.show();

        AsyncHttpClient client = new AsyncHttpClient();
        client.get("http://192.168.1.102/androidimg/wsblob.php", new AsyncHttpResponseHandler() {
            @Override
            public void onSuccess(int statusCode, Header[] headers, byte[] responseBody) {
                if (statusCode == 200){
                    progressDialog.dismiss();
                    try {
                        JSONArray jsonArray = new JSONArray(new String(responseBody));
                        for (int i=0;i<jsonArray.length();i++){
                            imagen.add(jsonArray.getJSONObject(i).getString("imagen"));
                        }
                        listView.setAdapter(new ImagenAdapter(getApplicationContext()));
                    }catch (JSONException e){
                        e.printStackTrace();
                    }
                }
            }

            @Override
            public void onFailure(int statusCode, Header[] headers, byte[] responseBody, Throwable error) {

            }
        });
    }
    private class ImagenAdapter extends BaseAdapter{
        Context ctx;
        LayoutInflater layoutInflater;
        ImageView imageView;
        Bitmap photo;

        public ImagenAdapter(Context applicationContext){
            this.ctx=applicationContext;
            layoutInflater = (LayoutInflater)ctx.getSystemService(LAYOUT_INFLATER_SERVICE);
        }

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

        @Override
        public Object getItem(int position) {
            return position;
        }

        @Override
        public long getItemId(int position) {
            return position;
        }

        @Override
        public View getView(int position, View convertView, ViewGroup parent) {
           ViewGroup viewGroup = (ViewGroup)layoutInflater.inflate(R.layout.activity_main_item, null);

            imageView=(ImageView)viewGroup.findViewById(R.id.imagen1);
            byte[] byteData = Base64.decode(imagen.get(position).toString(), Base64.DEFAULT);
            photo = BitmapFactory.decodeByteArray( byteData, 0,
                    byteData.length);
            imageView.setImageBitmap(photo);
            return viewGroup;
        }
    }
}