Gravatar is a very popular avatar image service (the name stands for Globally Recognized AVATAR). It uses a very simple algorithm to associate an image with a user email. Thus, a client application (Redmine in our case) sends a request with the hash of the user's email to this service and it returns the associated image. If no image is associated with the specified hash, Gravatar returns one of the default images (we reviewed them in The Display tab subsection of The general settings section in Chapter 3, Configuring Redmine).
Tip
Using avatars helps to visually identify your data (issues, comments, activities, and so on) among other user data.
This simplicity caused Gravatar to be chosen as the source of profile pictures in WordPress and StackOverflow. With custom plugins, support for Gravatars can also be added to Drupal, Joomla, SugarCRM, and so on. This means that, if you configure a Gravatar for Redmine, you will also have it automatically in WordPress, StackOverflow, some Drupal...