<div class="parent">
    <div>
        <h1>404 Page not found!</h1>
    </div>
    <div class="inner-container">
        <div class="image-container"></div>
    </div>
</div>

.parent {
    width: 100vw;
    height: 100vh;
    display: flex;
    flex-direction: column;
}

.inner-container {
    height: 100%;
    padding: 1em;
    box-sizing: border-box;
}

.image-container {
    background-image: url(/img/tank-girl.jpg);
    background-size: contain;
    background-repeat: no-repeat;
    background-position: center;
    height: 100%;
    box-sizing: border-box;
}