[@netweb Keymaster 12 years, 6 months ago]

You can select from various default avatars included with WordPress in the WordPress Dashboard -. Settings -> Discussion (/wp-admin/options-discussion.php)

Or look at adding one of the many avatar plugins:

https://wordpress.org/plugins/search.php?q=avatar and/or https://wordpress.org/plugins/search.php?q=avatar+bbpress

I don’t mind editing core files if that is what needs to be done.

This should never be done, when WordPress or bbPress has an update you will lose the changes you made.