var towelsURL = "imgs/towels1.jpg";

if (document.images) {
     var towels1 = new Image();
     towels1.src = "imgs/towels1.jpg";

     var towels2 = new Image();
     towels2.src = "imgs/towels2.jpg";
}

function changeTowels() {
     if (document.images) {
        if (towelsURL == "imgs/towels1.jpg") towelsURL = "imgs/towels2.jpg";
		
         document.myTowels.src = towelsURL;
	}
}