When you've logged into your Community account, you can change your avatar by changing your profile settings.
Click on your temporary profile icon
Click on 'Profile'
Click 'Edit profile'
Click 'change' and you should be able to upload a new avatar
Press the down arrow in the top right
Press on the placeholder avatar:
Press on 'profile'
Press on 'edit profile'
Press on 'Change' where the placeholder avatar resides and upload a new avatar: