The picture is generated by Gravatar.com based on an MD5 hash of your e-mail address. You can change it by:

(a) Using a different e-mail address (valid, though, or you risk hitting spam-filters) – this will give you a different “monster”

(b) Registering your identity on gravatar.com – this will let you choose what image to show there (so you could, for example, show a Splurg)