On all WordPress sites Gravatars are linked from Wordpres.com based on the email address used to sign up to an account on that site. Pretty ridiculous, I know. Info is here: http://codex.wordpress.org/Using_Gravatars
There are lots of modules that provide a workaround using your Facebook, Twitter or Google image. Or letting you upload one like http://wordpress.org/plugins/simple-local-avatars/
