Swing - paintComponent not called

Hi all,

For some reason the paintComponent is never called to print the
image. The image is valid, i have tested it with setIconImage(image)
and it worked fine. Why is the paintComponent never called? Any help
would be appreciated. Thanks in Advance.

The ProgressFrame is called from a run method, which is in a class
that implements Runnable. Which is called by creating a new thread.

Here is the code for the ProgressFrame:

public class ProgressFrame extends JFrame {

    public ProgressFrame()
    {
        Container contentPane = this.getContentPane();
        URL url = this.getClass().getResource("/Resources/busy.jpg");
        Image image = Toolkit.getDefaultToolkit().getImage(url); //new File
("/Resources/busy.jpg");

        DrawingPanel panel = new DrawingPanel(image);
        contentPane.add(panel);
        setTitle("Loading");
        setSize(384, 230);
        setResizable(false);
        setVisible(true);
        toFront();
    }
}

class DrawingPanel extends JPanel
{
    Image img;

    DrawingPanel (Image img)
    { this.img = img; }

    public void paintComponent (Graphics g) {
        super.paintComponent (g);
        g.drawImage (img, 0, 0, this);
    } // paintComponent
}