var imageURL = [
    "/img/page_top_0.gif",
    "/img/page_top_1.gif"
]
var images = [];
for (var i=0; i<imageURL.length; i++) {
    images[ imageURL[i] ] = new Image();
    images[ imageURL[i] ].src = imageURL[i];
}

function backToTop() {
    var x1 = x2 = x3 = 0;
    var y1 = y2 = y3 = 0;
    if (document.documentElement) {
        x1 = document.documentElement.scrollLeft || 0;
        y1 = document.documentElement.scrollTop || 0;
    }
    if (document.body) {
        x2 = document.body.scrollLeft || 0;
        y2 = document.body.scrollTop || 0;
    }
    x3 = window.scrollX || 0;
    y3 = window.scrollY || 0;
    var x = Math.max(x1, Math.max(x2, x3));
    var y = Math.max(y1, Math.max(y2, y3));
    window.scrollTo(Math.floor(x / 2), Math.floor(y / 2));
    if (x > 0 || y > 0) {
        window.setTimeout("backToTop()", 25);
    }
}

function changeImage(elem, src) {
    elem.src = (images[src] ? images[src].src : src);
}