preloadFlag = false;

function newImage(imgSrc) {
        if(document.images) {
                res = new Image();
                res.src = imgSrc;
                return res;
        }
}

function preloadImages() {
        if(document.images) {
                preloadFlag = true;
                btn_nl_o = newImage("_img/btn_nl_f2.gif");
                btn_fr_o = newImage("_img/btn_fr_f2.gif");
                btn_en_o = newImage("_img/btn_en_f2.gif");
        }
}

function swapImage (img,newSrc) {
        if(document.images && (preloadFlag == true)) {
                theImg=document.getElementById(img);
                theImg.setAttribute("src",newSrc);
        }
}