Other

import greenfoot.*;

public class MyWorld extends World
{
    public MyWorld()
    {
        super(800, 600, 1);
        /**  CREATE AND ADD AN OBJECT INTO WORLD */
        Actor object = getActor(); // to reference an Actor object
        addObject(object, 400, 300); // actor is put in world
    }
    
    // doesn't matter what type of Actor object is used
    /**  DON"T ACTUALLY USE THE FOLLOWING */
    // just returns an Actor object
    private Actor getActor()
    {
        return new Actor(){};
    }
}

setImage(new GreenfootImage(...));