Hey Wolf. Tried to get this to work in the beta to no avail...
However, tried again recently and it worked. Here's the process I used; Created account with gravatar with the email I want linked (should be the same email that you use for battlelog or your avatar won't show up there). After establishing an account, I simply chose to upload pic from my hard drive (any size, gravatar will modify as necessary), crop to your liking and save (make pic primary), and ten minutes later it was live.
Hope this helps