Browse for image file and display it using Java Swing

Create an ImageViewer with method like ImageViewer.setImage(Image), display the image in a JLabel.

ImageViewer

import java.awt.*;
import java.awt.event.*;
import java.awt.image.BufferedImage;
import javax.swing.*;
import javax.swing.border.EmptyBorder;
import java.util.Random;

public class ImageViewer {
    
    JPanel gui;
    /** Displays the image. */
    JLabel imageCanvas;

    /** Set the image as icon of the image canvas (display it). */
    public void setImage(Image image) {
        imageCanvas.setIcon(new ImageIcon(image));
    }
    
    public void initComponents() {
        if (gui==null) { 
            gui = new JPanel(new BorderLayout());
            gui.setBorder(new EmptyBorder(5,5,5,5));
            imageCanvas = new JLabel();
            
            JPanel imageCenter = new JPanel(new GridBagLayout());
            imageCenter.add(imageCanvas);
            JScrollPane imageScroll = new JScrollPane(imageCenter);
            imageScroll.setPreferredSize(new Dimension(300,100));
            gui.add(imageScroll, BorderLayout.CENTER);
        }
    }
    
    public Container getGui() {
        initComponents();
        return gui;
    }
    
    public static Image getRandomImage(Random random) {
        int w = 100 + random.nextInt(400);
        int h = 50 + random.nextInt(200);
        BufferedImage bi = new BufferedImage(
                w,h,BufferedImage.TYPE_INT_RGB);
        
        return bi;
    }

    public static void main(String[] args) throws Exception {
        Runnable r = new Runnable() {
            @Override
            public void run() {
                JFrame f = new JFrame("Image Viewer");
                // TODO Fix kludge to kill the Timer
                f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
            
                final ImageViewer viewer = new ImageViewer();
                f.setContentPane(viewer.getGui());
                
                f.pack();
                f.setLocationByPlatform(true);
                f.setVisible(true);

                ActionListener animate = new ActionListener() {

                    Random random = new Random();
                    
                    @Override
                    public void actionPerformed(ActionEvent arg0) {
                        viewer.setImage(getRandomImage(random));
                    }
                };
                Timer timer = new Timer(1500,animate);
                timer.start();
            }
        };
        SwingUtilities.invokeLater(r);
    }
}

Each time a new image is selected, you're creating components unnecessarily and in error here:

public void setTarget(File reference) {
    //....
    panel_1.setLayout(new BorderLayout(0, 0));
    panel_1.add(new JLabel(new ImageIcon(targetImg))); 
    setVisible(true);

Instead I would recommend that you have all these components created from the get-go, before any file/image has been selected, and then in this method, create an ImageIcon from the Image, and then simply use this Icon to set the Icon of an already existng JLabel rather than a new JLabel. This is done simply by calling myLabel.setIcon(new ImageIcon(targetImg));