You can only change your profile picture if your account is connected to a social network. If your account is linked to Facebook or Google+, your profile picture will be synced with the picture from those accounts.
Signed in with email? If you sign in using your email, your profile picture will display your initials and cannot be changed.
Want a profile picture? If you'd like to have a custom profile picture, simply link your account with Facebook or Google+ through the Settings Page.