$(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");

});