gistfile1.js

  if (!('backgroundSize' in document.body.style)) {
    var body = $$('body').first(), 
    img = body.getStyle('background-image').sub(/url\("?(.+?)"?\)/,'#{1}') || 'default.jpeg';
    body.setStyle('background:#ccc');
    var fill = new Element('img', {
      src: img,
      alt: ''
      }).setStyle('position:absolute; top:0; left:0; min-width: 100%; height: ' + ($(document.body).getHeight() + 80) + 'px; z-index:-1');
    body.insert({top: fill});
    Event.observe(window, 'load', function(){
      fill.setStyle('height:' + ($(document.body).getHeight() + 80) + 'px');
    });
    Event.observe(window, 'resize', function(){
      fill.setStyle('height:' + ($(document.body).getHeight() + 80) + 'px');
    });
  }