var currentDivNumber = 1;

function showDiv(divNumber) {
  if (divNumber == 0) {
     divNumber = currentDivNumber + 1;
     if (divNumber > numberOfPics) {
        divNumber = 1;
     }
  }
  currentDivName = "pic_" + currentDivNumber;
  currentAnchorName = "a_" + currentDivNumber;
  divName = "pic_" + divNumber;
  anchorName = "a_" + divNumber;

  document.getElementById(currentDivName).style.visibility = "hidden";
  document.getElementById(currentAnchorName).className = "";
  document.getElementById(divName).style.visibility = "visible";
  document.getElementById(anchorName).className = "selected";
  currentDivNumber = divNumber;
}