var sixURL = "imgs/six1.jpg";

if (document.images) {
     var six1 = new Image();
     six1.src = "imgs/six1.jpg";

     var six2 = new Image();
     six2.src = "imgs/six2.jpg";
}

function changeSix() {
     if (document.images) {
        if (sixURL == "imgs/six1.jpg") sixURL = "imgs/six2.jpg";
		
         document.mySix.src = sixURL;
	}
}