Try it as below:

If you want it specifically on homepage, insert the below code in `script editor/content editor webpart`.

But if you want it for entire site collection, add it in the custom css file referred in masterpage or on the masterpage itself.

    #s4-workspace
    {
    	background:url('../Style Library/Images/bg.png') no-repeat center center fixed;
    }

or as below:

    body.ms-backgroundImage
    {
      background: url('../Style Library/Images/bg.png') no-repeat center center fixed; 
    }

**Edit 2 :**

As per your comments, you want it for a specific section. Try it as below:

1) Edit your page by clicking on gear icon in top right.

[![enter image description here][1]][1]

2) Click on Add webpart. After doing that, click on the Script editor webpart and then click OK as below:

[![enter image description here][2]][2]

3) I have selected Script editor webpart. Inside the webpart, add the below code snippet.

    <script src="https://ajax.googleapis.com/ajax/libs/jquery/1.11.0/jquery.min.js"></script>
    <script src="https://sitecollectionurl/siteassets/parallax.min.js"></script>
    
    <style>
    .parallax-window {
        min-height: 400px;
        background: transparent;
    }
    </style>
    
    <div class="parallax-window" data-parallax="scroll" data-image-src="https://sitecollectionurl/SiteAssets/Instant%20Intranet/Theme%20Two/images/rapidcircle.png"></div>


4) Once that is done, click on Save. Checkin and publish the page. And there you have parallax for a specific section as below:

[![enter image description here][3]][3]

You need to change the path to that of your site collection.

  [1]: https://i.sstatic.net/Gm4mz.png
  [2]: https://i.sstatic.net/SQ1o8.png
  [3]: https://i.sstatic.net/vlQUp.png