var images;
var path_img_small="";
var path_img_big  ="";
var big_image     ="";

function SetBigImage(id)
  {
   var id_big  =document.getElementById('bg_img');
   var id_small=document.getElementById('small_img_'+id);
   if(id_big && id_small)
     {
      var new_big_img=images[id];
      images[id]     =big_image;
      big_image      =new_big_img;
      //---
      id_big.src=path_img_big+big_image;
      id_small.src=path_img_small+images[id];
     }
  }