We use Gravatar to manage avatars.
If you don't yet have a Gravatar, you can create an account and upload an image. Gravatar is tied to your email address, so you'll need to add the email you use on Crossbeam in order for it to work.
If you have a Gravatar, and it's linked to the email address you use on Crossbeam, it will populate your Crossbeam avatar automatically.
You can do this on your User Settings page.