BuddyPress.org

seobrien –
Read this: http://wpmu.org/how-to-add-a-custom-default-avatar-for-buddypress-members-and-groups/

iamnorthwind –
If you’re using another plugin to upload avatars, you’ll need to override the way BuddyPress fetches avatars.

Specifically, you’ll need to override the “bp_core_fetch_avatar” filter located in /buddypress/bp-core-avatars.php file.

Need a primer on filters? Read this:
https://codex.wordpress.org/Plugin_API#Filters