if (document.images)
{
img1on = new Image();      
img1on.src = "images/home_on.gif"; 
img2on = new Image(); 
img2on.src = "images/products_on.gif";  
img3on = new Image();
img3on.src = "images/location_on.gif";
img4on = new Image();
img4on.src = "images/contact_on.gif";

img1off = new Image(); 
img1off.src = "images/home_off.gif"; 
img2off = new Image();          
img2off.src = "images/products_off.gif"; 
img3off = new Image();
img3off.src = "images/location_off.gif";
img4off = new Image();
img4off.src = "images/contact_off.gif";
        }
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");
        }
}