It's an identicon that can be replaced with a gravatar.
To register a gravatar see here > http://en.support.wordpress.com/avatars/gravatars/
Gravatar images are cached and frequently take several days to display after a change. Do not keep uploading! The first place they tend to display is here in the forums and I see your Gravatar here. If several days have passed and that change is not in evidence then post here for Gravatar support > http://en.gravatar.com/site/contact-us/
To set up Gravatar display on your own blog see the bottom of this page > Settings > Discussion scroll down to "Avatars"
Go here > Settings > Discussion and scroll down to the bottom of the page. look for "Default Avatar
Default Avatar For users without a custom avatar of their own, you can either display a generic logo or a generated one based on their e-mail address."