if (window.addEventListener) {
  addEHandler = function(obj, eventName, handler) {
    obj.addEventListener(eventName, handler, false);
  };
  removeEHandler = function(obj, eventName, handler) {
    obj.removeEventListener(eventName, handler, false);
  };
}
else if (window.attachEvent) {
  addEHandler = function(obj, eventName, handler) {
    obj.attachEvent('on'+eventName, handler);
  };
  removeEHandler = function(obj, eventName, handler) {
    obj.detachEvent('on'+eventName, handler);
  };
}

addEHandler(window, 'load', doOnload);

var popup, curLeft, interval;

function doOnload()
{
popup = document.getElementById('showimage');
//  popup = document.getElementById('popup');
  curLeft = document.body.offsetWidth;
  popup.style.left = curLeft.toString() + 'px';
  interval = setInterval(movePopup, 50);
}

function movePopup()
{
  if (curLeft >= document.body.offsetWidth / 2) {
    curLeft -= 20;
    popup.style.left = curLeft.toString() + 'px';
  }
  else {
    clearInterval(interval);
  }
}