It would only take one or two random choices to produce added effect.
Try this for the Dot constructor:
public Dot()
{
setImage(new GreenfootImage(15, 15));
getImage().drawRect(Greenfoot.getRandomNumber(2)==0?img.getWidth()-2:0, Greenfoot.getRandomNumber(2)==0?img.getHeight()-2:0, 1, 1);
}
2013/9/5
2013/9/5
2013/9/5
2013/9/5
2013/9/6