When you've logged into your Community account, you can change your avatar by changing your profile settings.
On desktop:
Click on your temporary profile icon
Click on 'Profile'
Click 'Edit profile'
Click 'change' and you should be able to upload a new avatar
On mobile:
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:
