function getElementsByClass(node,searchClass,tag) {
  var classElements = new Array();
  var els = node.getElementsByTagName(tag); // use "*" for all elements
  var elsLen = els.length;
  var pattern = new RegExp("\\b"+searchClass+"\\b");
  for (i = 0, j = 0; i < elsLen; i++) {
  if ( pattern.test(els[i].className) ) {
      classElements[j] = els[i];
      j++;
    }
  }
  return classElements;
}

function pics(obj, pic) {
  document.getElementById('mainPic').src = pic; // nastavi hlavni obrazek na velkou zvetseninu
   
  var el = getElementsByClass(document,'active','span'); // nashromazdi vsechny sipky
  for(i = 0;i < el.length;i++) {
    el[i].style.display = 'none'; // necha zmizet vsechny sipky
  }
  
  obj.getElementsByTagName('span')[0].style.display = 'block'; // zobrazi konkretni sipku
  return false;
}