html, body {
  width: 100%;
  height: 100%;
  margin: 0;
  overflow: hidden;
}

.fullscreen-img {
  position: fixed;
  top: 0;
  left: 0;
  width: 100vw;
  height: 100vh;
  object-fit: cover;     /* WICHTIG */
  object-position: center;
}