You can do it by creating a div class call it bg for example:
.bg {
/* The image used */
background-image: url("image.png");
/* Center and scale the image nicely */
background-position: center;
background-repeat: no-repeat;
background-size: cover;
}
Now that we have set the div class we need to use it
<body>
<div class="bg">
/* place page contents here */
</div>
</body>