You need to use CSS.

<div class="background">
  <!-- Stuff on top of the background goes here. -->
</div>
<style>
.background {
  background-image: url('theImage.jpg');

  height: 100%; 

  background-attachment: fixed;
  background-position: center;
  background-repeat: no-repeat;
  background-size: cover;
}

body, html {
  height: 100%;
}
</style>

And then INSIDE the div you add what you want.

It should work.

So to do this, in your style.css add:

body {
  background: url(img.png);
}

But make sure img.png is an actual image (replace img.png with the image of your choice).

Also, to make sure the image is full-size, read this:

html { 
  background: url(img.png) no-repeat center center fixed;
  background-size: cover;
}

Read more about backgrounds here and here

maybe [this] (https://www.w3schools.com/html/html_images_background.asp) or [this] (https://www.w3schools.com/cssref/pr_background-image.asp) can help