Set background image to Body tag in CSS. 

body {
    background-image: url('link_image');
}

Example Source:

<!doctype html>
<html lang="en">
 <head>
  <meta charset="utf-8">
  <meta name="description" content="Support Code CSS: Add background image to body.">
  <meta name="author" content="bnson">
  <title>Support Code CSS: Add background image to body.</title>

  <style>
   body  {
    background-image: url('https://www.planwallpaper.com/static/images/518151-backgrounds_W3qVPu2.jpg');
   }
  </style>
 </head>

 <body>
   <!-- Content -->
 </body>

</html>

Set background image to Body tag in CSS.