function swap(targetId,buttonId,imgId){
  
  if (document.getElementById)
        {
        target = document.getElementById(targetId);
        buttonName = document.getElementById(buttonId);
            if (target.style.display == "none")
                {
                target.style.display = "";
                } 
            
            else 
                {
                target.style.display = "none";
                }
                
            if (target.style.display == "none")
                {
                buttonName.src = "images/nav/nav_" + imgId + "_dark.jpg";
                }
            else 
                {
                buttonName.src = "images/nav/nav_" + imgId + "_light.jpg";
                }
        }
}