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.

mylo2h2s

DP ALTerminator adds missing or empty ALT tags to your content images.
Thanks to various shortcodes such as <code>%%DP_TITLE%%</code> or <code>%%DP_DATE%%</code> it’s pattern can be easily customized (you can mix plain text elements with shortcodes to create complex patterns)
DP ALTerminator works when ALT tags are missing or when they are empty. Sometimes WordPress itself, visual editors, and so on, leave an ugly <code>alt=""</code> which is useless and not always recognized by other famous ALT tag managers.
DP ALTerminator works only with content images ($content variable), because I didn’t want to alter “extra” images, like widgets, logos, menu icons and so on (most of the times, you want to manually edit their ALT or leave it empty)

DP ALTerminator – Missing ALT manager

Adds ALT tags to your content images when they're missing or empty. It's pattern can be customized with simple shortcodes.

Gravatar SEO vs DP ALTerminator – Missing ALT manager