function cssPreloadImages()
{
    for(var i = 0; i < document.styleSheets.length; i++) {
        var pos    = document.styleSheets[i].href.lastIndexOf("/");
        var cssDir = (pos != -1) ?
                     document.styleSheets[i].href.substring(0, pos + 1) : "";
        var rules  = document.styleSheets[i].cssRules ?
                     document.styleSheets[i].cssRules :
                     document.styleSheets[i].rules;
        for ( var j = 0; j < rules.length; j++) {
            var style = rules[j].style;
            if (style.backgroundImage.toLowerCase().substr(0,4) == "url(") {
                var filename = style.backgroundImage.substring(4,
                                             style.backgroundImage.length - 1);
                if(filename.indexOf("http://") != 0 &&
                    filename.indexOf("/") != 0)
                {
                    filename = cssDir + filename;
                    var img = new Image();
                    img.src = filename;
                }
            }
        }
    }
}

if (window.attachEvent) {
    // IE
    window.attachEvent("onload", cssPreloadImages);
} else {
    // DOM
    window.addEventListener("load", cssPreloadImages, false);
}