A simple plugin that display images as layers of a parallax effect.
First of all, generate independent .PNG files of each layer of your image. This is the boring step.
There is a new post type called "Parallax". You can find it in the "Media" menu. A "Parallax" post is meant to receive the layers : the .PNG images we were talking about. Simply drag your files into the "Upload Layers" box. By the way folks, my WP is in french but the plugin is in english.
After they loaded, make sure that they will display in the right order : from the frontground to the background. You can order them by giving them a number in the tiny form next to the "Show". Set your original picture as featured image, set the intensity of the effect, and finally allow horizontal and/or vertical control.
Insert the effect in a post, using the shortcode [parallax id="ID_OF_YOUR_PARALLAX_POST"].