see the errors in your code in order to compile

Code:
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;

public class BT68Gui extends JFrame{
    
    private JComboBox box;
    private JLabel picture;
    
    private static String[] filename = {"b.png, x.png"};// and Array with two elements this Array stores the filename
    private Icon[] pics = {new ImageIcon(getClass().getResource(filename[0])), new ImageIcon(getClass().getResource(filename[1]))}; // This Array stores the pictures filename 0 is the first element of the Aray
    
    public BT68Gui(){
        super("The Title");
        setLayout(new FlowLayout());
        
            
        box = new JComboBox(filename);         
        box.addItemListener(
                new ItemListener(){
                    public void itemStateChanged(ItemEvent event){ //od  <--error
                    if(event.getStateChange()==ItemEvent.SELECTED)
                        picture.setIcon(pics[box.getSelectedIndex()]);// error
                    }
               }
       );   //error                   //compiler shows error
       add(box);       // compiler shows error
       picture = new JLabel(pics[0]);
       add(picture);
  }
}