var aBanners, iIdx;

function initBanners() {
  aBanners = $("#top div.banner");
  var iLen = aBanners.length;
  iIdx = Math.floor(Math.random()*iLen)
  showBanner( iIdx );

  function runTimer() {
    t = setTimeout( function(){
      iIdx = iIdx < iLen-1 ? iIdx+1 : 0;
      showBanner( iIdx );
      runTimer();
    }, 12000 );
  }
  runTimer();
}
function showBanner( idx ) {
  $(aBanners).hide();
  $(aBanners[ idx ]).show();
}