var preloaded = new Array();
function preloadImages() {
    for (var i = 0; i < arguments.length; i++){
    	preloaded[i] = document.createElement('img');
        preloaded[i].setAttribute('src',arguments[i]);
    };
};
preloadImages(
	'/wp-content/themes/usgs/images/nav1Hover.jpg',
	'/wp-content/themes/usgs/images/nav2Hover.jpg',
	'/wp-content/themes/usgs/images/nav3Hover.jpg',
	'/wp-content/themes/usgs/images/nav4Hover.jpg'
);
YAHOO.util.Event.addListener(window, 'load', preloadImages);


function swapImage() {
	var imageId = this.getAttribute('id');
	this.src = "/wp-content/themes/usgs/images/" + imageId + "Hover" + ".jpg";
}

function swapImageBack() {
	var imageId = this.getAttribute('id');
	this.src = "/wp-content/themes/usgs/images/" + imageId + ".jpg";
}

YAHOO.util.Event.addListener('nav1', 'mouseover', swapImage);
YAHOO.util.Event.addListener('nav2', 'mouseover', swapImage);
YAHOO.util.Event.addListener('nav3', 'mouseover', swapImage);
YAHOO.util.Event.addListener('nav4', 'mouseover', swapImage);

YAHOO.util.Event.addListener('nav1', 'mouseout', swapImageBack);
YAHOO.util.Event.addListener('nav2', 'mouseout', swapImageBack);
YAHOO.util.Event.addListener('nav3', 'mouseout', swapImageBack);
YAHOO.util.Event.addListener('nav4', 'mouseout', swapImageBack);