Where it says your name in the very top right hand corner next to the minimize, restore down, and close buttons, if you click on that you see your account name and your profile picture.
How do you change that picture?
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Sign up to join this communityJust click on the image again. There's a "change" link over the whole picture.
You can also change it on Gmail, if you have registered for it. This answer explains the process in detail.