Set background

import greenfoot.*;  // (World, Actor, GreenfootImage, Greenfoot and MouseInfo)
import java.awt.Color;   
/**
 * Write a description of class StartMenu here.
 * 
 * @author (your name) 
 * @version (a version number or a date)
 */
public class StartMenu extends World
{
     StartGame sg = new StartGame();
     //Instruc in = new Instruc(); 
    /**
     * Constructor for objects of class StartMenu.
     * 
     */
    public StartMenu()
    {    
        // Create a new world with 600x400 cells with a cell size of 1x1 pixels.
        super(600, 400, 1);
        
         setBackground(new GreenfootImage("l1.png"));
        
        {
            addObject(sg,getWidth()/2, getHeight()/2 );
        }
       {
            addObject(in,300, 100);
       }
    }
   
}

java.lang.IllegalArgumentException: java.io.FileNotFoundException: Could not find file: l1.png
	at greenfoot.GreenfootImage.loadFile(GreenfootImage.java:281)
	at greenfoot.GreenfootImage.<init>(GreenfootImage.java:108)
	at StartMenu.<init>(StartMenu.java:22)
	at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
	at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62)
	at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
	at java.lang.reflect.Constructor.newInstance(Constructor.java:423)
	at greenfoot.core.Simulation.newInstance(Simulation.java:617)
	at greenfoot.localdebugger.LocalDebugger$QueuedInstantiation.run(LocalDebugger.java:155)
	at greenfoot.core.Simulation.runQueuedTasks(Simulation.java:502)
	at greenfoot.core.Simulation.maybePause(Simulation.java:305)
	at greenfoot.core.Simulation.runContent(Simulation.java:218)
	at greenfoot.core.Simulation.run(Simulation.java:211)
Caused by: java.io.FileNotFoundException: Could not find file: l1.png
	at greenfoot.util.GreenfootUtil.getURL(GreenfootUtil.java:551)
	at greenfoot.GreenfootImage.loadFile(GreenfootImage.java:278)
	... 12 more