if (document.images) {            // Active Images
            img1on = new Image();      
            img1on.src = "vor.gif";

            img2on = new Image();      
            img2on.src = "nae.gif";

            img1off = new Image(); 
            img1off.src = "vorhl.gif";

            img2off = new Image(); 
            img2off.src = "naehl.gif";
        }

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

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