window.onload=window.onresize=window.onscroll=function()
{
  showImg();
};
function getPos(obj)
{
  var l=0;
  var t=0;
  while(obj)
  {
    l+=obj.offsetLeft;
    t+=obj.offsetTop;
    obj=obj.offsetParent;
  }
  return {left: l, top: t};
}
function showImg()
{
  var aTmg=document.getElementsByTagName('img');
  var scrollTop=document.documentElement.scrollTop||document.body.scrollTop;
  var scrollBottom=scrollTop+document.documentElement.clientHeight;
  for(var i=0;i<aTmg.length;i++)
  {
    var p=getPos(aTmg[i]);
    if(p.top<scrollBottom)
    {
      //alert(aTmg);
      aTmg[i].src=aTmg[i].getAttribute('_src');
    }
  }
}