responsivePostImg.html

<div class="postbox">

	<img src="http://placehold.it/500x400/" width="500" height="400" alt="Placehold.it" />

</div> <!-- /.postbox -->

responsivePostImg.js

function responsivePostImg() {

	var h = $(window).height()-110;
	var ih = $('.postbox img').attr('height');
	var iw = $('.postbox img').attr('width');
	var ratio = iw/ih;
	//console.log(ratio);
	$('.postbox').css({maxWidth:h*ratio});

}

$(document).ready(function(){

	responsivePostImg();
	
});	

$(window).resize(function(){
	
	responsivePostImg();
	
});