knuten

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.
<h3>A brief Markdown Example</h3>
<pre><code>[retina normal="path-to-normal-image" retina="path-to-retina-image" width="image-width" height="image-height" /]
</code></pre>

Retina for WP

This plugin let you add a shortcode with two different image sources one too a normal sized image and one for retina.

Jonathan Desrosiers

Slocum Studio

Seamlessly switches out images on Retina and other high pixel density screens with @2x versions.
The switch happens server side saving on HTTP requests.
No action is required on your part other than to activate it and to regenerate your thumbnails with the <a href="https://wordpress.org/extend/plugins/regenerate-thumbnails/" rel="ugc">Regenerate Thumbnails</a> plugin.
Plugin automatically:
<ul>
<li>Detects a HighDPI screen such as a Retina display</li>
<li>Detects all image sizes added using add_image_size() and adds @2x versions.</li>
<li>Replaces featured thumbnails and content images with the @2x versions for users with HighDPI screens.</li>
</ul>

Simple WP Retina

Seamlessly replaces images with @2x versions on Retina and other high pixel density screens. Change is performed server side saving requests.

Retina for WP vs Simple WP Retina