x = "images/nav_"
if (document.images) {            
 
contacton = new Image();  contacton.src = x + "contact_mo.jpg";
homeon = new Image();  homeon.src = x + "home_mo.jpg";
portfolioon = new Image();  portfolioon.src = x + "portfolio_mo.jpg";
serviceson = new Image();  serviceson.src = x + "services_mo.jpg";
 
contactoff = new Image();  contactoff.src = x + "contact_on.jpg";
homeoff = new Image();  homeoff.src = x + "home_on.jpg";
portfoliooff = new Image();  portfoliooff.src = x + "portfolio_on.jpg";
servicesoff = new Image();  servicesoff.src = x + "services_on.jpg";


        }
    function imgOn(imgName) {
            if (document.images) {
                document[imgName].src = eval(imgName + "on.src");       
            }
    }
    function imgOff(imgName) {
            if (document.images) {
                document[imgName].src = eval(imgName + "off.src");        
            }
           
    }