var f = false;
var b;
function prel(){

   	if(document.images) {


               b = new Image();
               b.src = "i/r.gif";
}
            	f = true;
}

function on(im){
	if (document.images && (f == true)) 
		{
		 document.images[im].src = "i/r.gif";}
		
}
function off(im){
	if (document.images && (f == true))
		document.images[im].src = "i/s.gif";
}