var n = 5;
var f = false;
var b = new Array;
function prel(){

   	if(document.images) {
	  for(i=1;i<=n;i++)
               {str = 'i/p' + i + '_o.gif';
               b[i-1] = new Image();
               b[i-1].src = str;
}
            	f = true;
 		}
}
function on(im){
	if (document.images && (f == true)) 
		{
		 document.images[im].src = "images/"+im+"_o.gif";}
		
}
function off(im){
	if (document.images && (f == true))
		document.images[im].src = "images/"+im+".gif";
}