delaying the loading of a certain css background image

...
#big-menu {
    background: #fff url(../images/big-menu.jpg) no-repeat;
}
#some-menu {
    background: #fff url(../images/some-menu.jpg) no-repeat;
}
#some-other-menu {
    background: #fff url(../images/some-other-menu.jpg) no-repeat;
}
...

function loadBG(){
    document.getElementById("big-menu").style.backgroundImage="../images/big-menu.jpg";
}
window.onload=loadBG();

var imgGroup = new YAHOO.util.ImageLoader.group(window, 'load', 2);

$('firstElement').addClass('some-menu');
$('secondElement').addClass('some-other-menu');
$('lastElement').addClass('big-menu');

$('firstElement').css("background-image", "some-menu.jpg");
$('secondElement').css("background-image", "some-other-menu.jpg");
$('lastElement').css("background-image", "big-menu.jpg");