$(document).ready(function() {
    bg();
    $(window).resize(bg);
    $('#bg').mantle({ mantleBgImage: "images/dian.gif" });
});
function bg() {
    var bg = $('#bg');
    var image = new Image();
    $(image).load(function() {
        var h = $(window).height() - 26;
        var w = image.width * h / image.height;
        bg.height(h);
        bg.width(w);
    }).attr('src', bg.attr('src'));
}