This plugin creates the image files required by the Retina (or any high-DPI) devices and displays them to your visitors accordingly. Your website will look beautiful and sharp on every device. The retina images will be generated for you automatically (you can also do it manually) and served to the visitors. You will be able to control everything from a nice dashboard.
The Pro version adds support for Retina for Full-Size, support for Lazy-Loading for the Responsive Images, Upload & Media Replace for your Full-Size or for your Retina for Full-Size, and a few more options.
Check the official page about the plugin here: WP Retina 2x.