var compatible = (document.getElementsByTagName && document.createElement);
if(compatible) {
  initPage();
}

function initPage(){
  var x = document.getElementsByTagName('img');
  for(var i=0;i<x.length;i++){
    if(x[i].className != 'thumb') continue;
	x[i].onclick = function(){
                     document.mainpic.src=this.src;
                   }
	x[i].onfocus = function(){
                     document.mainpic.src=this.src;
                   }
  }
}