var n = 6;

var f = false;

var b = new Array;

   	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 = "i/"+im+"_o.gif";}

		

}

function off(im){

	if (document.images && (f == true))

		document.images[im].src = "i/"+im+".gif";

}