<picture>
  <source srcset="https://images.unsplash.com/photo-1484656551321-a1161420a2a0?ixlib=rb-1.2.1&ixid=eyJhcHBfaWQiOjEyMDd9&auto=format&fit=crop&w=1950&q=80" media="(prefers-color-scheme: dark)">
  <img src="https://images.unsplash.com/photo-1472457847783-3d10540b03d7?ixlib=rb-1.2.1&auto=format&fit=crop&w=3300&q=80">
</picture>

* {
  margin: 0;
  padding: 0;
}
body {
  display: flex;
  height: 100vh;
  align-items: center;
  justify-content: center;
}
img { max-height: 100vh; }