Super simple.
Log into your account.
If you're already logged into your coaching account, navigate to 'My Accounts' from the drop down menu in the top right.
Once in 'My Accounts', head back to the top right and you'll see your coach profile menu.
Click here and choose 'My Profile'
Once here, you'll see a section at the top to update your avatar image.
This will then automatically apply in all the accounts you own or have been invited to join in your 'My Accounts' listing.
Thanks
FITR