JSFiddle - React, Tailwind, and code Playground

HTML

<img src="http://www.hdwallpapersfan.com/wp-content/uploads/2013/10/mountain-4.jpg" onload="this.classList.add('show')">

CSS

img {
    opacity: 0;
    -webkit-transition: 1000ms opacity;
    -moz-transition: 1000ms opacity;
    -ms-transition: 1000ms opacity;
    -0-transition: 1000ms opacity;
    transition: 1000ms opacity;
}

img.show {
    opacity: 1
}