JSFiddle - React, Tailwind, and code Playground

HTML

<img src="https://www.gettyimages.co.uk/gi-resources/images/CreativeLandingPage/HP_Sept_24_2018/CR3_GettyImages-159018836.jpg" alt="" id="smart_thumbnail" class="">

CSS

img{
  transition: all 0.3s ease-in-out;
  width:50%;
  cursor:pointer;
}

.big{
  width:100%;
}

JavaScript

var thumbnailElement = document.getElementById('smart_thumbnail')

thumbnailElement.addEventListener("click", function() {
	if(thumbnailElement.classList.contains("big")){
  	thumbnailElement.classList.remove("big")
  } else {
  	thumbnailElement.classList.add("big")
  }
})