var pic_current = 0;

function switchpic() {
  var target = document.getElementById('pic');
  var next = document.getElementById('next');
  next.onclick = function () { target.src = pics[nextpic()]; }
  var prev = document.getElementById('prev');
  prev.onclick = function () { target.src = pics[prevpic()]; }
}

onload_items[onload_items.length] = switchpic;

function nextpic() {
  pic_current++;
  if (pics.length <= pic_current) pic_current = 0;
  return pic_current;
}

function prevpic() {
  pic_current--;
  if (0 > pic_current) pic_current = pics.length-1;
  return pic_current;
}