WordPress/jQuery Before After Image Slider
This plugin allows you to show and compare two pictures side by side. You can use it as a WordPress-Plugin or standalone (requires jQuery and jQuery UI Slider).
Select from 4 different modes (Overlay, From Left, From Right, Side-by-Side). It contains a shortcode for WordPress and can easy be customized using the contained SCSS file.
Usages are for instance showing before and after or old and new images directly together.