var gymURL = "imgs/gym1.jpg";

if (document.images) {
     var gym1 = new Image();
     gym1.src = "imgs/gym1.jpg";

     var gym2 = new Image();
     gym2.src = "imgs/gym2.jpg";
	 
	 var catalog = new Image();
	 catalog.src = "imgs/catalog.jpg";
	 
}

function changeGym() {
     if (document.images) {
        if (gymURL == "imgs/gym1.jpg") gymURL = "imgs/gym2.jpg";
		else if (gymURL == "imgs/gym2.jpg") gymURL = "imgs/catalog.jpg";
		else if (gymURL == "imgs/catalog.jpg") gymURL = "imgs/gym1.jpg";
		
         document.myGym.src = gymURL;
	}
}

function changeGymBack() {
		if (document.images) {
			if (gymURL == "imgs/gym1.jpg") gymURL = "imgs/catalog.jpg";
			else if (gymURL == "imgs/gym2.jpg") gymURL = "imgs/gym1.jpg";
			else if (gymURL == "imgs/catalog.jpg") gymURL = "imgs/gym2.jpg";
			
			document.myGym.src = gymURL;
		}
}