$(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");

});