function showNextSlide(layerId, numSlides) {
  var timeOut  = 5000;
  var curSlide = document.getElementById(layerId).style.backgroundImage;
  var numPos   = curSlide.lastIndexOf('.') - 2;
  var slideNum = curSlide.substr(numPos, 2);
  slideNum = 1 * slideNum + 1;
  if (slideNum > numSlides) {
    slideNum = 1; 
  }
  if (slideNum < 10) {
   slideNum = "0" + slideNum;
  }
  curSlide = curSlide.substr(0, numPos) + slideNum + curSlide.substr(numPos + 2)
  document.getElementById(layerId).style.backgroundImage = curSlide;
  var interv = setTimeout("showNextSlide('" + layerId + "', '" + numSlides + "')", timeOut);
}

function startShow(layerId, numSlides) {
  var interv = setTimeout("showNextSlide('" + layerId + "', '" + numSlides + "')", 5000);
}