JRootPaneの背景として画像を表示

JFrame frame = new JFrame("@title@") {
  @Override protected JRootPane createRootPane() {
    JRootPane rp = new JRootPane() {
      // private final TexturePaint texture = makeCheckerTexture();
      @Override protected void paintComponent(Graphics g) {
        super.paintComponent(g);
        Graphics2D g2 = (Graphics2D) g.create();
        g2.setPaint(texture);
        g2.fillRect(0, 0, getWidth(), getHeight());
        g2.dispose();
      }

      @Override public void updateUI() {
        super.updateUI();
        BufferedImage bi = makeBufferedImage("test.jpg");
        setBorder(new CentredBackgroundBorder(bi));
        setOpaque(false);
      }
    };
    return rp;
  }
};
// frame.getRootPane().setBackground(Color.BLUE);
// frame.getLayeredPane().setBackground(Color.GREEN);
// frame.getContentPane().setBackground(Color.RED);
((JComponent) frame.getContentPane()).setOpaque(false);
frame.setJMenuBar(createMenubar());
frame.getContentPane().add(new MainPanel());