You don't need any JavaScript to get that effect.
Simplified example:
HTML:
<header>
<h1>Cats</h1>
</header>
<body>
<p>Lorem ipsum dolor sit amet, consectetur adipisicing elit. Laborum magnam eaque harum ullam earum accusantium praesentium quia a expedita voluptas inventore porro odit libero aliquid sunt culpa voluptatem! Nesciunt commodi?</p>
...
</body>
CSS:
header {
background: url("//placekitten.com/800/500") black;
background-size: 100% auto;
background-repeat: no-repeat;
padding: 15% 0;
}