wadadanet

Just by uploading foo@2x.png, resized 1x are created automatically
<h4>How to use</h4>
<ul>
<li>When used for posting</li>
</ul>
<ol>
<li>Upload file with ‘@2x’ in file name</li>
<li>Select ‘size item’ as ‘1x size’ when inserting media</li>
</ol>
<ul>
<li>PHP 
echo(wp_get_attachment_image( IMAGE_ID,”1x” ));</li>
</ul>

WP Retina Image

Just by uploading foo@2x.png, resized 1x are created automatically

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.

WP Retina Image vs Simple WP Retina