if (document.images) {

    img1on = new Image();
    img1on.src = "images/nav/purchase_over.gif";
    img2on = new Image();
    img2on.src = "images/nav/products_over.gif";
    img3on = new Image();
    img3on.src = "images/nav/care_over.gif";

    img1off = new Image();
    img1off.src = "images/nav/purchase_off.gif";
    img2off = new Image();
    img2off.src = "images/nav/products_off.gif";
    img3off = new Image();
    img3off.src = "images/nav/care_off.gif";

}

function imgOn(imgName) {
        if (document.images) {
            document[imgName].src = eval(imgName + "on.src");
        }
}

function imgOff(imgName) {
        if (document.images) {
            document[imgName].src = eval(imgName + "off.src");
        }
}