Gravatar images in WordPress sites don`t have alt and title tags, with this plugin you can resolve this problem on your site.

Gravatar SEO

A light and simple plugin to insert automatically ALT & TITLE tags into Gravatar images in your site.

leofontin

Fill in the alt attributes of images is very important for the SEO of your website. 
With Image Alt Editor, you can quickly fill in all the alt attributes of your images. 
Very practical, fast and easy to use.

Image Alt Editor

Quickly edit the ALT attributes of your images and medias

Gravatar SEO vs Image Alt Editor