$(document).ready(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");
		
});