import javax.swing.*;
import java.awt.*;
import java.awt.image.BufferedImage;

public class Foo {
    private static BufferedImage img;

    public static void main(String[] args) {
        loadImage();

        JFrame f = new JFrame();
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        f.setContentPane(createContentPane());

        f.pack();
        f.setVisible(true);
    }

    private static JPanel createContentPane() {
        JPanel p = new JPanel() {
            @Override
            protected void paintComponent(Graphics graphics) {
                super.paintComponent(graphics);

                if (img != null) {
                    graphics.drawImage(img, 0, 0, null);
                }
            }

            @Override
            public Dimension getPreferredSize() {
                if (img != null) {
                    return new Dimension(img.getWidth(), img.getHeight());
                }
                return super.getPreferredSize();
            }
        };

        p.add(new JButton("Button"));
        return p;
    }

    private static void loadImage() {
        // Hier dein Bild laden, z.B. per ImageIO.read(File f)
        img = new BufferedImage(300, 300, BufferedImage.TYPE_INT_RGB);
    }
}