JSFiddle - React, Tailwind, and code Playground

HTML

<div class="overlay">
    <div class="helper"></div>
    <img src="http://dummyimage.com/550x480/000/fff?text=H/V Centered and height/width variable">
</div>

CSS

.helper {
    height:50%;
    width:100%;
    margin-bottom:-240px;
    min-height: 240px;
}
.overlay {
    width: 100%;
    height: 100%;
    position: fixed;
    top: 0px;
    left: 0px;
    cursor: pointer;
    background-color: rgba(0, 0, 0, 0.5);
}
img {
    margin: 0 auto;
    max-width: 100%;
    max-height: 100%;
    display: block;
}

JavaScript

/*
The only problem with this is when the width is very small the image is no longer vertically centered.
*/