This plugin let you add a shortcode with two different image sources one too a normal sized image and one for retina. The plugin will then load the right image depending on if it’s a retina display or not.
For more info and how to use it.
A brief Markdown Example
[retina normal="path-to-normal-image" retina="path-to-retina-image" width="image-width" height="image-height" /]
Cleaned up the code a bit and added some info.