macOS Pictures on Simple Java GUI won't work.

import java.awt.FlowLayout;
import java.awt.event.ActionListener;
import java.awt.event.ActionEvent;
import javax.swing.JFrame;
import javax.swing.JButton;
import javax.swing.Icon;
import javax.swing.ImageIcon;
import javax.swing.JOptionPane;

public class Gui extends JFrame{
	
	private JButton reg;
	private JButton custom;
	
	public Gui(){
		super("The title");
		setLayout(new FlowLayout());
		
		reg = new JButton("reg Button");
		add(reg);
			
		Icon b = new ImageIcon(getClass().getResource("b.png"));
		Icon x = new ImageIcon(getClass().getResource("x.png"));
		custom = new JButton("Custom", b);
		custom.setRolloverIcon(x);
		add(custom);
		
		HandlerClass handler = new HandlerClass();
		reg.addActionListener(handler);
		custom.addActionListener(handler);
	}
	
	private class HandlerClass implements ActionListener{
		public void actionPerformed(ActionEvent event) {
			JOptionPane.showMessageDialog(null, String.format("%s", event.getActionCommand()));
		}
		
	}

}


Main Class:

import javax.swing.JFrame;
class apples{
	public static void main(String[] args){
		
		Gui go = new Gui();
		go.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
		go.setSize(300,200);
		go.setVisible(true);
		
	}
}