var n = 13;
var f = false;
var b = new Array;
function prel(){

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