Changing Background Image Of JPanel When Certain Condition Meets?

In my program one of the panel has a background image and I need that background image to be updated/refresh or changed whenever the panel reaches certain number of clicks

here's the code

Constructor/gui
import java.awt.*;
import javax.swing.*;
import javax.swing.text.*;;
 public class demo { 
JFrame frame = new JFrame ("Idle Game Test!");
JPanel backGroundPanel = new JPanel ();

[code].....

also is there other way to access those static variables? the way i implemented them works but i do think there are other ways that are more suitable or better but i cant figure out.