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

function changeImagesArray(array) {
    if (document.images && (preloadFlag == true)) {
        for (var i = 0; i < array.length; i += 2) {
            document[array[i]].src = array[i + 1];
        }
    }
}

function changeImages() {
    changeImagesArray(changeImages.arguments);
}

function toggleImages() {
    for (var i = 0; i < toggleImages.arguments.length; i += 2) {
        if (selected == toggleImages.arguments[i]) changeImagesArray(toggleImages.arguments[i + 1]);
    }
}

var selected = '';
var preloadFlag = false;
function preloadImages() {
    if (document.images) {
        sale_over = newImage("images/sale-on-furniture-over.jpg");
      guarantee_over = newImage("images/secure-shopping-over.jpg");
      cart_over = newImage("images/view-shopping-basket-over.png");
      check_over = newImage("images/check-out-over.png");
      preloadFlag = true;
    }
}


<!-- Begin
function curriculum_open() {
    window.open("school-curriculum.htm","","height=500,width=550,left=80,top=60");
}
function staff_open() {
    window.open("school-staff.htm","","height=200,width=550,left=80,top=60");
}
function progress_open() {
    window.open("progress-reports.htm","","height=240,width=550,left=80,top=60");
}
function transportation_open() {
    window.open("transportation.htm","","height=130,width=550,left=80,top=80");
}
function health_open() {
    window.open("health.htm","","height=170,width=550,left=80,top=90");
}
function care_open() {
    window.open("daycare.htm","","height=280,width=550,left=80,top=60");
}