<div class="header-image">
    <img src="smesh-colour-hires.png">
</div>

.header-image {
    background-image: url("animals-deep-ocean-deep-sea-130621.jpg");
    background-size: cover;
    background-position: center;
    height: 500px;
    margin: auto;
    display:flex;
    align-items:center;
    justify-content:center;
}

.header-image img{
    max-width: 800px;
}