I just set up a profile on Gravatar, which was easy since it made use of my existing WordPress.com account.
Gravatar is a service that automatically adds a profile photo any time you comment or post on a blog/website/forum that supports it. You create an account, register your e-mail address (or multiple addresses), and upload a photo. After that, your photo will appear whenever you use your e-mail address when commenting/posting. This includes sites (like mine) that require commenters to provide an e-mail address, but do not display it to the public.