var urlArray = new Array(3);
var banArray = new Array(3);
var counter = 1;

//add your necessary URL's
urlArray[1] = "preview.php?foto=data/images/koszary1.jpg";
urlArray[2] = "preview.php?foto=data/images/koszary2.jpg";
urlArray[3] = "preview.php?foto=data/images/koszary3.jpg";

if(document.images)
{
  for(i = 1; i < 4; i++)
  {
    banArray[i] = new Image(333, 229);
    banArray[i].src = "data/images/koszary" + i + "_.jpg";
  }
}


function changeBanner() //banner changer function
{
  if(counter > 3)
   counter = 1;

  document.banner.src = banArray[counter].src; //sets a new banner

  url = urlArray[counter]; //sets a new URL to the banner
  counter++; //increase the counter for the next banner
}

//change the timer as necessary (minutes * 60000) or (seconds * 1000)
var timer = window.setInterval("changeBanner()", 3000);