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 !

snagysandor

Parallax Scrolling Effect by Enllax.js
An ultra-lightweight and super easy to use plugin for applying parallax scrolling effect to any scrolling element. You can set parallax scrolling effects to background elements, and also in both directions (vertically or horizontally)
<a href="https://nagysandor.org/enllax-js-demo" rel="nofollow ugc">Demo</a>
Shortcode example
With external image: 
[enllax xclass=”enllax” offset=”0″ direction=”horizontal” speed=”0.8″ img=”image url”] 
Your content. 
[enllaxend]
With internal image (featured image and content): 
[enllax_post postid=”1″ xclass=”enllax” offset=”0″ direction=”horizontal” speed=”0.8″]
xclass: CSS class name (default: enllax) 
offset: background offset in pixel (default: 0) 
direction: vertical (default) or horizontal 
speed: to allow background image to move with different speed, just use a data attribute with a numeric value as a multiplier for scrolling speed, less is slower and 1 is normal (default value: 0.5) 
img: Parallax background image url

Image Parallax vs Parallax Scrolling Enllax.js