FolioVision

There’s one problem with Gravatars. They are very slow to load. Each page with comments on them makes one call per comment to the Gravatar server. While a single call takes only a 100ms, on a page with hundreds of comments, we are talking about major slowdowns. Page loads can take 20 seconds and more.
This plugin solves this problem by:
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caching gravatars with WordPress cron job..
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caching gravatars on comment submission
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maintaining a single copy of the default gravatar instead of downloading it again and again for all the email addresses with no gravatar associtated
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<a href="http://foliovision.com/seo-tools/wordpress/plugins/fv-gravatar-cache/installation" rel="nofollow ugc">Installation guide</a>
<a href="http://foliovision.com/seo-tools/wordpress/plugins/fv-gravatar-cache" rel="nofollow ugc">Support and more information</a>

FV Gravatar Cache

I deactivated plugin but still contains elements in Database

Database Junk

Speeds up your website by making sure the gravatars are stored on your website and not loading from the gravatar server.

Julio Potier

This plugin takes place between the gravatar picture and the default avatar return. If an email is not associated with a gravatar account, a Google Image request is sent to grab the first best relevant face picture of this member.
When a google image photo have been chosen, the user will receive a mail with 3 links, 1st to delete this avatar if not relevant enought, 2nd to refresh it, if its email is not associated with a gratavar account, 3rd an invitation to create a gravatar account.
Also, you can add you own avatar by uploading your picture, easily !
Then, you can use a gravatar cache to keep all avatar files on your server and, limit the HTTP request on gravatar.com
Here comes a (french, just watch ;p) demo video, 15mn length. (video = version 1.8 !) 
http://www.youtube.com/watch?v=90mBwyNd5zU

Gravatar Google Image

Add a step between "no gravatar account" and "default gravatar sent back", using Google Images Search Service + 2 bonus usage

FV Gravatar Cache vs Gravatar Google Image