<img class="fullscreen" src="https://picsum.photos/id/1016/2000/1000" alt="Pretty Pic">

html,
body {
  margin: 0;
  padding: 0;
}
.fullscreen {
  display: block;
  width: 100%;
  height: 100vh;
  object-fit: cover;
  object-position: top;
}