There’s two immediate options – BP Better Registration includes avatar upload in the registration process and BuddyPress Force Profile Photo https://buddydev.com/plugins/bp-force-profile-photo/ sets the site so users cannot access it unless they have a profile photo set.
during initial registration, WordPress has yet to create a user_id for the member, so can not save the avatar, only after the user has confirmed their email address will WordPress be able to save the avatar and reference it to that user.
Envato Market has a $20 plugin that claims to “add upload avatar to registration page.”
It adds it to the registration page, but the avatar doesn’t get saved; it’s a scam.
Never buy anything for your Buddypress site from Envato Market – I made a $20 mistake by not reading about the company first.
Listen to Venutius; he is totally correct about what your pluggable options currently are. (Unless you create your own plugin, finding a way to temporarily save the image until after the account is created, and then associate it to the profile, but good luck with all that!)