带有背景图像的面板

我有以下代码:

class ImagePanel extends JPanel {

          private Image img;

          public ImagePanel(String img) {
            this(new ImageIcon(img).getImage());
          }

          public ImagePanel(Image img) {
            this.img = img;
            Dimension size = new Dimension(img.getWidth(null), img.getHeight(null));
            setPreferredSize(size);
            setMinimumSize(size);
            setMaximumSize(size);
            setSize(size);
            setLayout(null);
          }

          public void paintComponent(Graphics g) {
            g.drawImage(img, 0, 0, null);
          }
        }

以及以下内容:

public GUIVenDetails() throws MalformedURLException, IOException{

        mapPanel = new ImagePanel("http://www.netstate.com/states/symb/gamebirds/images/wild_turkey.jpg");
        mapPanel.setPreferredSize(new Dimension(200,200));
        mapPanel.setMinimumSize(mapPanel.getPreferredSize());
        mapPanel.setMaximumSize(mapPanel.getPreferredSize());


        add(mapPanel);
    }

public static void main(String[] args) throws MalformedURLException, IOException, XPathExpressionException, SAXException, ParserConfigurationException {


        GUIVenDetails gui = new GUIVenDetails();

        Dimension screenSize = Toolkit.getDefaultToolkit().getScreenSize();
        Dimension windowSize = gui.getSize();

        int windowX = Math.max(0, (screenSize.width  - windowSize.width ) / 2);
        int windowY = Math.max(0, (screenSize.height - windowSize.height) / 2);

        JFrame f=new JFrame();
        f.setSize(new Dimension(400,800));
        f.setLocation(windowX, windowY); 

        f.add(gui);
        f.setVisible(true);
    }

当我运行代码时,我只看到白色。。。这是为什么?