JSFiddle - React, Tailwind, and code Playground

HTML

<div id="content">
<img src="http://www.nasa.gov/images/content/66676main_image_feature_221_jw4.jpg">
<img src="http://lamininbeauty.co.za/images/products/loader.gif">
</div>

CSS

.small_img{border:1px solid green;}

JavaScript

window.onload = function() {
   var n=document.getElementById('content').getElementsByTagName('img'), i=n.length;
   while(i--){n[i].className = n[i].clientWidth > 400 ? 'large_img' : 'small_img' ;}
};