var sportURL = "imgs/sport1.jpg";

if (document.images) {
     var sport1 = new Image();
     sport1.src = "imgs/sport1.jpg";

     var sport2 = new Image();
     sport2.src = "imgs/sport2.jpg";
	 
	 var catalog = new Image();
	 catalog.src = "imgs/catalog.jpg";
	 
}

function changeSport() {
     if (document.images) {
        if (sportURL == "imgs/sport1.jpg") sportURL = "imgs/sport2.jpg";
		else if (sportURL == "imgs/sport2.jpg") sportURL = "imgs/catalog.jpg";
		else if (sportURL == "imgs/catalog.jpg") sportURL = "imgs/sport1.jpg";
		
         document.mySport.src = sportURL;
	}
}

function changeSportBack() {
     if (document.images) {
        if (sportURL == "imgs/sport1.jpg") sportURL = "imgs/catalog.jpg";
		else if (sportURL == "imgs/sport2.jpg") sportURL = "imgs/sport1.jpg";
		else if (sportURL == "imgs/catalog.jpg") sportURL = "imgs/sport2.jpg";
		
         document.mySport.src = sportURL;
	}
}