JSFiddle - React, Tailwind, and code Playground

HTML

<div class="wrap">
    <nav class="menu">
        <ul>
            <li>Something</li>
        </ul>
    </nav>
    <div class="container"></div>
</div>

CSS

body {
    margin: 0;
}

.wrap {
    width: 100%;
}

.menu {
    top: 0;
    position: fixed;
    background-color: green;
    width: 100%;
}

.container {
    background-image: url(http://www.spektyr.com/PrintImages/Cerulean%20Cross%203%20Large.jpg);
    background-size: cover;
    padding-bottom: 100%; /* <- This value should be equal to height / width */
}