$(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(function() {
	        resizeBg();
	}).trigger("resize");
});