Dougal Campbell

This plugin allows you to automatically add Gravatars for commenters to your 
theme, if your theme does not already support them.
According to the Gravatar.com website, Gravatars are Globally Recognized 
Avatars, or an “avatar image that follows you from weblog to weblog 
appearing beside your name when you comment on gravatar enabled sites.” 
You register with the Gravatar server, and upload an image which you will 
use as your avatar. The gravatar image is keyed to your email address, so 
that it is unique to you.
This plugin will display gravatars for the people who comment on your posts. 
You do not need to modify any of your template files — just activate the 
plugin, and it will add gravatars to your comments template automatically.
<h3>Credits</h3>
Based on a code snippet from Matt Mullenweg: 
 http://photomatt.net/2007/10/20/gravatar-enabled/ 
 http://pastebin.ca/743979
Props to David Potter for pointing out that Gravatar normalizes email 
addresses to lowercase before hashing with MD5: 
 http://dpotter.net/Technical/index.php/2007/10/22/integrating-gravatar-support/

Easy Gravatars

nice

Add Gravatars to your comments without modifying any template files. Just activate, and you're done!

Robert Chapin

Automatically replaces the standard resolution Gravatars with HiDPI (Retina) Gravatars using HTML (when supported) or Javascript (as needed).
You need this plugin if you want blog comments to look crisp and clear on Retina, HD, and similar devices!
You need this plugin if you want compatibility with all web browsers. The HiDPI features added in WordPress 4.2 are not compatible with older browsers, unless you have this plugin activated. The included Javascript helps make your website look the same in new and old browsers.
<h3>Theme Requirements</h3>
You may not omit the <a href="https://codex.wordpress.org/Function_Reference/wp_head" rel="nofollow ugc">wp_head</a> and <a href="https://codex.wordpress.org/Function_Reference/wp_footer" rel="nofollow ugc">wp_footer</a> template tags for this plugin to work correctly.
<h3>Cache Compatibility</h3>
HiDPI Gravatars is designed to be fully compatible with page caching plugins such as WP Super Cache.
Pages that were cached prior to activating HiDPI Gravatars will need to be refreshed. Empty the cache to make sure the new Gravatars will appear.
HiDPI Gravatars is not compatible with any Gravatar caching plugins.
<h3>Other Gravatar Plugins</h3>
HiDPI Gravatars might not detect customized Gravatar functions in other plugins. As of version 1.4, HiDPI Gravatars relies on the WordPress <a href="https://codex.wordpress.org/Function_Reference/get_avatar" rel="nofollow ugc">get_avatar</a> filter. Custom avatar generators that avoid or disable this filter will be ignored by HiDPI Gravatars.

HiDPI Gravatars

Enables high resolution Gravatar images on any browser that supports them.

Easy Gravatars vs HiDPI Gravatars