BuddyPress.org

Try this: upload an avatar file to the BuddyPress plugin folder here: buddypress/bp-core/images/

Now edit file bp-core-avatars.php located in the buddypress/bp-core/ folder , find line #141 and change the avatar file name on the end of the line per your needs, e.g.
Code:

$default_grav = apply_filters( ‘bp_core_mysteryman_src’, BP_PLUGIN_URL . ‘/bp-core/images/avatar.jpg’ );

Now login to your main blog, go to BuddyPress tab -> General Settings and be sure the Mystery Man is checked as the default avatar.

You’re done. You have now your default gravatar created by you