function newImage( imgSrc ) {
    var imgName = new Image();
    imgName.src = imgSrc;

    return imgName;
}

function change( imgName, imgLocation ) {
    document[imgName].src = imgLocation ;
}

var preloadFlag = false;
function loadImages() {
    if (document.images) {
        button_home    = newImage("/images/buttons/pushed_home.png");
        button_bio     = newImage("/images/buttons/pushed_biography.png");
        button_gallery = newImage("/images/buttons/pushed_gallery.png");
        button_contact = newImage("/images/buttons/pushed_contact.png");
        preloadFlag = true;
    }
}