w3schools background image cover code example

Example 1: css background full width

body { 
  background: url(img/bg-image.jpg) no-repeat center center fixed; 
  -webkit-background-size: cover;
  -moz-background-size: cover;
  -o-background-size: cover;
  background-size: cover;
}

Example 2: how to change background image in html

<style>
div {
  background-image: url('img_girl.jpg');
}
</style>