Full-screen responsive background image

http://css-tricks.com/perfect-full-page-background-image/

//HTML
<img src="images/bg.jpg" id="bg" alt="">

//CSS
#bg {
  position: fixed; 
  top: 0; 
  left: 0; 

  /* Preserve aspet ratio */
  min-width: 100%;
  min-height: 100%;
}

OR

img.bg {
  /* Set rules to fill background */
  min-height: 100%;
  min-width: 1024px;

  /* Set up proportionate scaling */
  width: 100%;
  height: auto;

  /* Set up positioning */
  position: fixed;
  top: 0;
  left: 0;
}

@media screen and (max-width: 1024px) { /* Specific to this particular image */
  img.bg {
    left: 50%;
    margin-left: -512px;   /* 50% */
  }
}

OR

//HTML
<img src="images/bg.jpg" id="bg" alt="">

//CSS
#bg { position: fixed; top: 0; left: 0; }
.bgwidth { width: 100%; }
.bgheight { height: 100%; }

//jQuery
$(window).load(function() {    

        var theWindow        = $(window),
            $bg              = $("#bg"),
            aspectRatio      = $bg.width() / $bg.height();

        function resizeBg() {

                if ( (theWindow.width() / theWindow.height()) < aspectRatio ) {
                    $bg
                        .removeClass()
                        .addClass('bgheight');
                } else {
                    $bg
                        .removeClass()
                        .addClass('bgwidth');
                }

        }

        theWindow.resize(resizeBg).trigger("resize");

});