knofte

Gravatar Alt Tags is a WordPress plugin designed to improve accessibility and SEO on your website. It automatically adds descriptive alt tags to Gravatar images based on the user’s display name. This feature aids screen readers and search engines by providing meaningful descriptions for Gravatar images.

Alt-Tags-for-Gravatar

A simple WordPress plugin that enhances website accessibility and SEO by adding alt tags to Gravatar images for post authors and commenters.

litefeel

Comments Avatar Lazyload at server side load replace the src property of img tag. It is relly lazyload. It was successfully checked by W3C. 
It is only lazyload comments avatar.
Related Links:
<ul>
<li><a href="https://www.litefeel.com/comments-avatar-lazyload/" title="Comments Avatar Lazyload Plugin for WordPress" rel="nofollow ugc">Plugin Homepage</a></li>
<li><a href="https://www.litefeel.com/" title="Author For Comments Avatar Lazyload Plugin" rel="nofollow ugc">Author Homepage</a></li>
<li><a href="https://github.com/lite3/comments-avatar-lazyload" title="On GitHub" rel="nofollow ugc">On GitHub</a></li>
</ul>

Comments Avatar Lazyload

Comments Avatar Lazyload at server side load replace the src property of img tag. It is relly lazyload. It was successfully checked by W3C.

Alt-Tags-for-Gravatar vs Comments Avatar Lazyload