var n = 8;
var f = false;
var b = new Array;

function prel(){
           if(document.images) {
          for(i=1;i<=n;i++)
               {str = 'img/p' + i + '_o.gif';
               b[i-1] = new Image();
               b[i-1].src = str;
}
                    f = true;
                 }
}
function on(im){
        if (document.images && (f == true))
                {ind = im.substring(3,1);
                 document.images[im].src = "img/"+im+"_o.gif";}
}

function off(im){
        if (document.images && (f == true))
                document.images[im].src = "img/"+im+".gif";
}