if (document.images) {
    img1 = new Image();
    img1.src = "img/last_on.png";
    img2 = new Image();
    img2.src = "img/menu_bck.png";
    img3 = new Image();
    img3.src = "checkbox/bck.png";
    img4 = new Image();
    img4.src = "checkbox/bck_laiem.png";
    img5 = new Image();
    img5.src = "checkbox/chk_on.png";
    img6 = new Image();
    img6.src = "img/loading_wh.gif";
}