WebMaestro.Fr

Upload the layers of your image, and create a parallax effect !
It works great on smart devices too !
The first step is to create with your favorite picture editing software (Photoshop, GIMP…) an independent image for each ground composing your image. All these layers shall be the same size, and should be in .png format as they are meant to include transparency areas. 
Then, on your post editing page in WordPress, click the Insert Media button, and (the same way you would create a gallery) upload your layers under the Create Parallax item. Reorganise the layers if needed, from the background to the front. 
You can finally configure the animation (calibrate, invert, limit, scalar, friction), and publish !
There is <a href="http://webmaestro.fr/image-parallax-plugin-wordpress/" rel="nofollow ugc">some examples</a> on the plugin page.
It uses <a href="http://wagerfield.github.io/parallax/" rel="nofollow ugc">parallax.js</a> by Matthew Wagerfield.

Image Parallax

Create images with a parallax effect. Upload some layers, configure the animation, and publish !

Demis Patti

flavor

cbParallax lets you add custom background images with a vertical or horizontal parallax effect to your WordPress site. You can set a single background for the entire site or configure individual backgrounds for each post, page, or product.
Features:
<ul>
<li>Background image with smooth parallax scrolling effect</li>
<li>One image for all pages or individual backgrounds per page/post</li>
<li>Works vertically and horizontally</li>
<li>Various overlay patterns to choose from</li>
<li>Color overlay with opacity control</li>
<li>Boxed layout option with customizable margins</li>
<li>Mobile device detection with option to disable parallax on mobile</li>
<li>Compatible with WooCommerce products</li>
</ul>
Requirements:
<ul>
<li>Your theme must support the WordPress <a href="https://developer.wordpress.org/themes/functionality/custom-backgrounds/" rel="nofollow ugc">Custom Backgrounds</a> theme feature</li>
<li>For parallax effect, images should be at least 1920px x 1200px</li>
<li>PHP 8.0 or higher</li>
</ul>
<h3>Privacy Policy</h3>
This plugin does not collect, store, or transmit any personal data. All settings are stored locally in your WordPress database.

cbParallax

Custom background images with parallax effect for posts, pages and products.

Image Parallax vs cbParallax