Your profile picture (or "avatar") is an image that represents you online - a little picture that appears next to your name when you interact with this site.
We use "Gravatars" - ("Globally Recognized Avatars"). You just need to upload your picture once, and then when you participate in any Gravatar-enabled site, your Gravatar image will automatically follow you there.
Gravatar is a free service and is automatically included in every WordPress.com account and is run and supported by Automattic. Note that whilst many sites and blogs do this, Gravatar isn't yet in use everywhere. Notable exceptions are services like Facebook, Twitter, and LinkedIn.
Visit the Gravatar website, create your free account and upload your picture. It is as simple as that!