$(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");

});