<body>
  <div class="day"></div>
  <div class="night"></div>
</body>
body {
  margin: 0;
  padding: 0;
}

div {
  position: absolute;
  height: 100vh;
  width: 100vw;
}

.day {
  background-image: url("https://images.unsplash.com/photo-1477959858617-67f85cf4f1df?ixlib=rb-1.2.1&ixid=eyJhcHBfaWQiOjEyMDd9&auto=format&fit=crop&w=2613&q=80");
  background-size: cover;
  background-repeat: no-repeat;
}

.night {
  background-image: url("https://images.unsplash.com/photo-1493540447904-49763eecf55f?ixlib=rb-1.2.1&ixid=eyJhcHBfaWQiOjEyMDd9&auto=format&fit=crop&w=2250&q=80");
  background-size: cover;
  background-repeat: no-repeat;
  clip-path: polygon(100vw 0, 0% 0vh, 100vw 100vh);
}

External CSS

This Pen doesn't use any external CSS resources.

External JavaScript

This Pen doesn't use any external JavaScript resources.