var selImg = 1;
 var prevImg = '';

 function switchImg(imgURL,x) {
  selImg = x;
  var imgObj = document.getElementById('img_main');
  imgObj.src = imgURL;
 }

 function showBigImage() {
  hideBigImage();
  document.getElementById('big_images_holder').style.display = 'inline';
  var curImg = "main_image_" + selImg;
  prevImg = document.getElementById(curImg)
  prevImg.style.display = 'inline';
 }

 function hideBigImage() {
  document.getElementById('big_images_holder').style.display = 'none';
  if(prevImg)
  { prevImg.style.display = 'none'; }
 }