We do not pull a picture based on information sent directly from Google OpenId (or any other OpenId other than Facebook).
We use the Gravatar service for our user images. If you edit your profile, you will see a "change picture" link underneath the picture. This link takes you to Gravatar's site.
If we are pulling that image for you, it means that the email you have entered in your user profile here is an email that is connected to your Gravatar account, and furthermore that the picture we are using is connected to that email address within Gravatar.
If you would like to use a different image, you will need to change the image associated with the email on Gravatar's side. If you want to use a different image here only, but not anywhere else that uses Gravatar, you could use +-addressing. You could add firstname.lastname@example.org to Gravatar and then add a special image there. This will leave anything pulling Gravatar for email@example.com unaffected.