body{
    -webkit-animation:zoom 5s infinite;
    animation:zoom 5s infinite;
    background:url(https://picsum.photos/1900/1900/?random) center center fixed no-repeat;
}
@-webkit-keyframes zoom{
    0%,100%{
        background-size:100%;
    }
    50%{
        background-size:125%;
    }
}
@keyframes zoom{
    0%, 100%{
        background-size:100%;
    }
    50%{
        background-size:125%;
    }
}

<style>
.zoom {
  padding: 50px;
  background-color: green;
  transition: transform .2s; /* Animation */
  width: 200px;
  height: 200px;
  margin: 0 auto;
}

.zoom:hover {
  transform: scale(1.5); /* (150% zoom)*/
}
</style>

<div class="zoom"></div>

#example1 {
  background: url(mountain.jpg);
  background-repeat: no-repeat;
  background-size: auto;
}

#example2 {
  background: url(mountain.jpg);
  background-repeat: no-repeat;
  background-size: 300px 100px;
}
/* One way */
background-size: cover;

/* Other option */
background-size: contain;