set background image in bootstrap 4 code example
Example 1: full page background image
html {
background: url(images/bg.jpg) no-repeat center center fixed;
-webkit-background-size: cover;
-moz-background-size: cover;
-o-background-size: cover;
background-size: cover;
}
Example 2: bootstrap create full screen background image
.wrapper{background: url('/assets/64531/green_suburb.jpg') no-repeat center center fixed;
-webkit-background-size: cover;
-moz-background-size: cover;
-o-background-size: cover;
background-size: cover;}