Btw: there's one small mistake:
if you want to use your own avatars instead of the mistery man, you have to use apostrophe before and after the url:

define ( 'BP_AVATAR_DEFAULT', 'http://example.com/default-avatar.jpg ');
define ( 'BP_AVATAR_DEFAULT_THUMB', 'http://example.com/default-avatar-thumb.jpg ');