different bg on section change

<div id="pg1">
<style>
#pg1 {
background-image: url("Your Image Here");
background-size: 25%;
background-position: center;
background-repeat: repeat-y-x;
position: fixed;
height: 100%;
width: 100%;
left: 0;
top: 0;
z-index: -1;
}
</style>
</div>