update avatar by external system

hi all!
I wish made this thing: I wish deny the users to upload or change her own avatar, and after, by an external program ( I know the id of the user) set the new avatar (atm suppose an url with the image): how can I force this thing (mantaining the good things of the xenforo avatar, like the dimensioning, etc...)
thanks again! :)
looking in depth i've seen in XenForo\ControllerPublic\Account.php the function
actionAvatarUpload, that seems the function that upload the avatar, update the db...
how can I call this function? thanks again ;)