Drawing partially transparent PNG onto JPanel

How would i go about drawing a partially transparent image onto a JPanel by overriding its void paint (Graphics g) method? I've tried the obvious way, which is to load the image and then use ((Graphics2D)g).drawImage(...) but that didn't work, and the internet isn't telling me much.

JLabel is capable of doing this, but if you want to do something else with the panel/image, the using paintComponent and Graphics#drawImage is also an option.

Using this as a base...

I was able to render this...

import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.EventQueue;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.LinearGradientPaint;
import java.awt.Point;
import java.awt.image.BufferedImage;
import java.io.File;
import java.io.IOException;
import javax.imageio.ImageIO;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.UIManager;
import javax.swing.UnsupportedLookAndFeelException;

public class TransparentPNG {

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

    public TransparentPNG() {
        EventQueue.invokeLater(new Runnable() {
            @Override
            public void run() {
                try {
                    UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
                } catch (ClassNotFoundException | InstantiationException | IllegalAccessException | UnsupportedLookAndFeelException ex) {
                }

                JFrame frame = new JFrame("Testing");
                frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                frame.setLayout(new BorderLayout());
                frame.add(new TestPane());
                frame.pack();
                frame.setLocationRelativeTo(null);
                frame.setVisible(true);
            }
        });
    }

    public class TestPane extends JPanel {

        private BufferedImage img;

        public TestPane() {
            try {
                img = ImageIO.read(new File("Pony.png"));
            } catch (IOException ex) {
                ex.printStackTrace();
            }
        }

        @Override
        public Dimension getPreferredSize() {
            return img == null ? new Dimension(200, 200) : new Dimension(img.getWidth(), img.getHeight());
        }

        @Override
        protected void paintComponent(Graphics g) {
            super.paintComponent(g);
            Graphics2D g2d = (Graphics2D) g.create();
            LinearGradientPaint lgp = new LinearGradientPaint(
                    new Point(0, 0),
                    new Point(0, getHeight()),
                    new float[]{0f, 1f},
                    new Color[]{Color.GREEN, Color.YELLOW});
            g2d.setPaint(lgp);
            g2d.fillRect(0, 0, getWidth(), getHeight());
            if (img != null) {
                int x = (getWidth() - img.getWidth()) / 2;
                int y = (getHeight() - img.getHeight()) / 2;
                g2d.drawImage(img, x, y, this);
            }
            g2d.dispose();
        }
    }
}

Check out How to use labels and Performing Custom Painting