function preload(filename){preload_image= new Image();preload_image.src = filename;}
function roll(name, filename){document.images[name].src = filename;}

preload('../Images/Navigation/nav_on_03.gif');
preload('../Images/Navigation/nav_on_04.gif');
preload('../Images/Navigation/nav_on_05.gif');
preload('../Images/Navigation/nav_on_06.gif');
preload('../Images/Navigation/nav_on_07.gif');