function initImages() {

        // preload images
        aboutus_btn_us =       new Image();
        aboutus_btn_us.src=    "pics/aboutus_img.jpeg";

        aboutus_btn_s =     new Image();
        aboutus_btn_s.src=  "pics/aboutus_roll.png";

        portfolio_btn_us =     new Image();
        portfolio_btn_us.src=  "pics/portfolio_img.jpeg";

        portfolio_btn_s =     new Image();
        portfolio_btn_s.src=  "pics/portfolio_roll.png";

        contact_btn_us =       new Image();
        contact_btn_us.src=    "pics/contact_img.jpeg";

        contact_btn_s =     new Image();
        contact_btn_s.src=  "pics/contact_roll.png";
}

function rollImg(imgDocID, imgObjName) {
        document.images[imgDocID].src = eval(imgObjName + ".src");
}