For a fixed centered image like a wallpaper or something simply go to your AdminCP -> Customization -> Themes and click on the </> icon.
Then click on the CSS tab, search for custom.css, copy & paste this code and click save.
body {
background: url("https://www.your-domain.com/path-to/your-image.jpg") no-repeat center center fixed;
-webkit-background-size: cover;
-moz-background-size: cover;
-o-background-size: cover;
background-size: cover;
}