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

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

var preloadFlag = false;
if (document.images) {
        pic1_over = newImage("tt-cowboy-roundup-brown.jpg");
        pic2_over = newImage("tt-cowboy-roundup-blue.jpg");
        pic3_over = newImage("tt-midnight-cowgirl.jpg");
        pic4_over = newImage("tt-saddles-n-stars.jpg");
        preloadFlag = true;
}