JSFiddle - React, Tailwind, and code Playground

HTML

<div class="imagesTest">
    <img class="bottom" src="http://www.romanianmonasteries.org/images/rodna_mountains10.jpg" />
    <img class="transition" src="http://www.geoglance.com/photo/62/62_mountains_300x200.jpg" />
</div>

CSS

.imagesTest {
    position:relative;
    height:200px;
    width:300px;
}
.imagesTest img {
    position:absolute;
    -webkit-transition: opacity 1s ease-in-out;
    -moz-transition: opacity 1s ease-in-out;
    -o-transition: opacity 1s ease-in-out;
    transition: opacity 1s ease-in-out;
}

.imagesTest img.transition:hover {
    opacity:0;
}