Gravatar is a service that can be used to associate an avatar (user image) with an e-mail address. Gravatar is a popular service supported by many websites allowing users to upload their image once and use it across those websites.
The openstreetmap.org website supports Gravatar - users can choose in their settings to use Gravatar for their e-mail address instead of a locally uploaded image.
The use of Gravatar is regarded as a severe privacy problem (includes publishing of your email address in obfuscated, easily decodable form to everyone) by some people. See the two links in this forum post and this website programmers' discussion.
For this reason changes to the OSM website (www.openstreetmap.org) have been made in September 2016 which result in gravatars being enabled only if you actually have an account with gravatar.com and no longer the old behaviour that turned them on for everybody (the code will re-check the settings when you change your e-mail address). Note that the code for help.openstreetmap.org was not effected by any of the changes and still generates and makes public the hash for your e-mail address (if you create an account there).
- Gravatar website provides instructions on how to manage your own images.