if (document.images) {
 image1on = new Image();
 image1on.src = "images/zoom2.jpg"
 image1off = new Image();
 image1off.src = "images/frankfurt3.jpg" 
}


counter = 3;

function changeImages() {

if (counter%2) {
document.image1.src = image1on.src;
 }

else {
document.image1.src = image1off.src;
 }

counter = counter + 1;

}