var briefURL = "imgs/brief1.jpg";

if (document.images) {
     var brief1 = new Image();
     brief1.src = "imgs/brief1.jpg";

     var brief2 = new Image();
     brief2.src = "imgs/brief2.jpg";
	 
	 var brief3 = new Image();
	 brief3.src = "imgs/brief3.jpg";
}

function changeBrief() {
     if (document.images) {
        if (briefURL == "imgs/brief1.jpg") briefURL = "imgs/brief2.jpg";
		else if (briefURL == "imgs/brief2.jpg") briefURL = "imgs/brief3.jpg";
		
         document.myBrief.src = briefURL;
	}
}