JSFiddle - React, Tailwind, and code Playground

HTML

<section id="portfolio">
  <img src="http://oi66.tinypic.com/330hitc.jpg">
  <img src="http://oi66.tinypic.com/330hitc.jpg">  
  <img src="http://oi66.tinypic.com/330hitc.jpg">  
  <img src="http://oi66.tinypic.com/330hitc.jpg">  
  <img src="http://oi66.tinypic.com/330hitc.jpg">  
  <img src="http://oi66.tinypic.com/330hitc.jpg">  
</section>

CSS

#portfolio {
  display: flex;
  flex-direction: column;
  align-items: center;
}

#portfolio img {
  border: 1px solid white;
  width: 100%; 
}

@media screen and (min-width: 768px) {   
  #portfolio {
    flex-direction: row;
    flex-wrap: wrap;
  }
  
  #portfolio img {
    width: 33%;
  }
}