html - How to change background image with the help of url parameter in php?

I'm trying to change background image of a simple php page with help url parameter. I'm very newbie and don't know much coding.

<img width="250" src="https://i.imgur.com/<?php echo $_GET['img']; ?>.png">
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When i use the above code in url i will get desire image from imgur.com by just entering the image link for example:

http://example.com/?img=frotzyG (full image url: https://i.imgur.com/frotzyG.jpg)

but it just show in a div e.g.

<div class="abc">
<img width="250" src="https://i.imgur.com/<?php echo $_GET['img']; ?>.png">
</div>
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Now i want the same thing to happen for background image. I have a simple php page and its background image source is located in a css (e.g stylesheet.css)

Here's CSS file code:

body {
  background: url(images/bg.jpg) no-repeat center center fixed; 
  -webkit-background-size: cover;
  -moz-background-size: cover;
  -o-background-size: cover;
  background-size: cover;
}
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How to change background image with the help of url parameter (or whatever you call it.)

P.S. Sorry for my bad English, hope you understand my point.