How do I display an image in JButton?

package org.kodejava.swing;

import javax.swing.ImageIcon;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.SwingUtilities;
import javax.swing.WindowConstants;
import java.awt.FlowLayout;

public class ButtonImageExample extends JFrame {
    public ButtonImageExample() {
        initComponents();
    }

    public static void main(String[] args) {
        SwingUtilities.invokeLater(() -> new ButtonImageExample().setVisible(true));
    }

    private void initComponents() {
        setTitle("My Buttons");
        setSize(500, 500);
        setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
        getContentPane().setLayout(new FlowLayout(FlowLayout.CENTER));

        // Creates two JButton object with an images to display. The image can be
        // a gif, jpeg, png and some other type supported. And we also set the
        // mnemonic character of the button for short-cut key.
        JButton okButton = new JButton("OK", new ImageIcon(
                this.getClass().getResource("/images/ok.png")));
        okButton.setMnemonic('O');
        JButton cancelButton = new JButton("Cancel", new ImageIcon(
                this.getClass().getResource("/images/cancel.png")));
        cancelButton.setMnemonic('C');

        getContentPane().add(okButton);
        getContentPane().add(cancelButton);
    }
}

JButton with Image Icon