jQuery(document).ready(function($){
$(window).load(function() {
$("#bg") .css("display", "block");
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(function() {
resizeBg();
}).trigger("resize");

});

});