The Stackexchange websites use gravatar. According to Gravatar's Terms of Service
The full Gravatar TOS is here
I went through Stackexchange.com/legal.
I found these two to be somewhat relevant
3) Subscriber Content
You agree that all Subscriber Content that You contribute to the Network is perpetually and irrevocably licensed to Stack Exchange under the Creative Commons Attribution Share Alike license.
7) Registration and security
As a condition to using Services, Subscriber may be required to register with Stack Exchange and select a password and profile name. Subscriber shall provide Stack Exchange with accurate, complete, and updated registration information, including Subscriber’s e-mail address
These two may be used to say that, since the user has given SE license to use its email, SE can use a hash of the email for the gravatar service. Thus freeing SE from making a reference to Gravatar's TOS.