var preloaded = new Array();
function preload_images() {
    for (var i = 0; i < arguments.length; i++){
        preloaded[i] = document.createElement('img');
        preloaded[i].setAttribute('src',arguments[i]);
    };
};
preload_images(
    'images/nav_01_on.gif',
	'images/nav_02_on.gif',
	'images/nav_03_on.gif',
	'images/nav_04_on.gif',
	'images/nav_05_on.gif'
);