How do I set contentPane without using the main method or calling other methods?

I understand that the following sets the background colour of the contentPane. How do I set a picture as its background instead?

I've tried these:

• Setting background images in JFrame
• JAVA: Ways to fill a Frame. add(), setContentPane(), getContentPane()
• http://www.dreamincode.net/forums/topic/131439-setting-background-color-content-pane/

But none of them have worked.

JLabel lblbackground = new JLabel();
lblbackground.setBounds(20, 20, 160, 160);
lblbackground.setBorder(new LineBorder(new Color(0, 0, 0), 2));
lblbackground.setIcon (new ImageIcon (this.getClass().getResource("/boundary/background.jpg")));
lblbackground.setHorizontalAlignment (SwingConstants.CENTER);                   
BufferedImage img = new BufferedImage(lblbackground.getIcon().getIconWidth(), lblbackground.getIcon().getIconHeight(), BufferedImage.TYPE_INT_RGB);
Graphics g = img.createGraphics();
lblbackground.getIcon().paintIcon(null, g, 0, 0);
g.dispose();
Image newing = img.getScaledInstance(150, 150, java.awt.Image.SCALE_SMOOTH);
lblbackground.setIcon(new ImageIcon(newing));           

//getContentPane().setLayout(new GridBagLayout());
contentPane = new JPanel();
//contentPane.setBackground(Color.LIGHT_GRAY);
contentPane.setBorder(null);
contentPane.setLayout(null);        
contentPane.add(lblbackground);
setContentPane (contentPane);

(...)
// 1) Create your image;
final ImageIcon image = new ImageIcon("../folder/myImage.gif");

//2) Create a JPanel with a background image;
  JPanel  myPanel = new JPanel(){
            @Override
            public void paintComponent(Graphics g)
            {
                g.drawImage(image.getImage(), 0, 0, null);
            }
    };

//3) Add panel 
getContentPane().add(myPanel);

(...)