// the field (outside any method, but inside the class)
private int imageNumber;
// your if block above, revised
if (Greenfoot.mouseClicked(this))
{
    MouseInfo mouse = Greenfoot.getMouseInfo();
    if (mouse.getClickCount() == 2)
    {
        imageNumber = 1-imageNumber; // toggle value between zero and one
        String suffix = "Edit"; // assume zero value
        if (imageNumber == 1) suffix = "Less"; // adjust if one value
        setImage(new GreenfootImage("German SdKfz 234_1 "+suffix+".png")); // set image
        if (imageNumber == 1) getImage().scale(75, 75); // scale if one value
    }
}

  if (a<2)
        { 
            if (atWorldEdge())
            {turn(20);
            }
            move(Greenfoot.getRandomNumber(20)+5);
         turn(Greenfoot.getRandomNumber(20)-10);
           if (canSee(apple.class))
           {
            eat(apple.class);
            a = a+1;
         }    
      }