Witam, jak ustawić w Javie obrazek w formacie .jpg jako tł w sytuacji, gdzie już mam podzielone okno na panele?

package gra;

import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;

public class okienko extends JFrame implements ActionListener {
   
    public okienko okienko;
    private JLabel bg;
    private JFrame frame;
    private JPanel gui,belka,pole,przyciski;
    private JButton B1,B2,B3,B4;
   

    public okienko()
    {    
        gui = new JPanel(new BorderLayout(3,3));
       
       // gui.setBorder(new EmptyBorder(5,5,5,5));
        gui.setPreferredSize(new Dimension(800,600));
        
        belka = new JPanel();
        belka.setBackground(Color.WHITE);
        belka.setPreferredSize(new Dimension(800,50));
        gui.add(belka,BorderLayout.NORTH);
        
        pole = new JPanel();
   
        pole.setPreferredSize(new Dimension(800,500));
        gui.add(pole,BorderLayout.CENTER);
        
        
        przyciski = new JPanel();
        przyciski.setBackground(Color.gray);
        przyciski.setPreferredSize(new Dimension(800,50));    
        B1 = new JButton("Start");
        przyciski.add(B1);
        B2 = new JButton("Wyniki");
        przyciski.add(B2);
        B3 = new JButton("Pomoc");
        przyciski.add(B3);
        B4 = new JButton("Reset");
        przyciski.add(B4);
        gui.add(przyciski,BorderLayout.SOUTH);
        
        
        frame = new JFrame("Hold The Ball");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setSize(800, 600);
        frame.setResizable(false);
        frame.setVisible(true);
        frame.add(gui);
        frame.pack();
    
    }
    @Override
    public void actionPerformed(ActionEvent e) {
      
    }

public class Gra_All {
    public static void main(String[] args) {
        okienko okno = new okienko();
    
    }
}

}