如何使图像大小适合 JFrame 大小?

I have a JPanel into a JFrame. I loaded a picture on the JPanel but its shown just a part of the picture: This is the part of the code where i did it:

JPanel panelImg = new JPanel()
{
    public void paintComponent(Graphics g)
    {
        Image img = new ImageIcon("Welcome.png").getImage();
        Dimension size = new Dimension(img.getWidth(null), img.getHeight(null));
        setPreferredSize(size);
        setMinimumSize(size);
        setMaximumSize(size);
        setSize(size);
        setLayout(null);
        g.drawImage(img, 0, 0, null);
    }
};
mainFrame.add(panelImg);

这就是它的样子:

全图是这样的:

有没有办法将图片缩放到 JFrame 的大小?提前致谢