фон для JFrame

java-Frame background.java

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

/**
 * Created by ezhov_da on 18.04.2018.
 */
class JFrameTest extends JFrame {
    private JLabel label;

    public JFrameTest() {
        setTitle("Example");
        setSize(400, 400);
        setLocationRelativeTo(null);
        setDefaultCloseOperation(EXIT_ON_CLOSE);

        setContentPane(new JLabel(new ImageIcon("image.jpg")));

        setLayout(new FlowLayout());
        label = new JLabel("Test");
        add(label);

        setVisible(true);
    }

    public static void main(String args[]) {
        new JFrameTest();
    }
}