changing BP Avatar from gravatar (to custom or even just mystery man!)

This is perhaps a fairly simple question, but I cannot find the answer to it. I originally set my default avatars as those classic videogame bit characters (with all the bright colors, which now look a bit garrish and do not match my site), and would like to change it from that. I cannot find where I originally set it, and am not sure how to even change it to Mystery Man. I'd like to change it to a custom avatar like in Sarah Gooding's article (https://premium.wpmudev.org/blog/how-to-add-a-custom-default-avatar-for-buddypress-members-and-groups/) but find how to get to the mystery man, to even get that custom image working.
Any ideas or guidance?