body {
    height: 100vh;
    display: flex;
    flex-direction: column;
}

img {
    display: block;
    margin: auto;
    max-width: 90vw;
    width: 640px;
    border-radius: 0 0 8px 8px;
}

iframe {
    flex-grow: 1;
    width: 100vw;
}