No! Don't give your portrait to Gravatar. Regardless of whether you trust Gravatar Corp, site owners can carelessly betray the identity of users posting under pseudonyms. A cautionary tale:
Hashim in Saudi Arabia secretly reads an American blog about homosexuality. One time, the blog discusses the Middle East, so Hashim comments describing his own experience. He is careful to give a false name 'John Elton' to protect his identity. However the blog software, Wordpress, also demands an email address. The software promises 'your email address will not be published'. Hashim trusts the blog owner and thinks nothing of typing his email address
email@example.com. There's no risk, his email doesn't even contain his real name
Two years later, Hashim signs up for Stack Overflow under his real name. He gives Gravatar his email and portrait.
Unbeknownst to Hashim , in 2011 Wordpress decided to install Gravatar on their platform, to make it 'more social'. Portraits are added to new comments, but also to millions of archived comments. (Wordpress didn't consider this a privacy issue, because email addresses remain secret). As a consequence, Hashim's face is now publicly displayed next to John Elton's story, but he isn't aware of this.
The secret police happen across the blog post, recognise Hashim from his portrait, and arrest him.