offscreen_preload.js

window.onload = function(){
    var doc = document
    , img = doc.createElement('img')
    , div = doc.createElement('div')
    , body = doc.getElementsByTagName('body')[0];
    img.className = 'preload';
    img.style.cssText = ';position:absolute;top:-999em;left:-999em;width:0;height:0;visibility:hidden;';
    // ajax loaded or predefined img list
    var arr = imgList
    , str
    , tmp;
    div.id = 'preload-wrapper';
    while(src = arr.pop()){
        tmp = img.cloneNode(!1);
        tmp.src = src;
        div.appendChild(tmp);
    }
    body.appendChild(div);
    // remove div when all loaded
}