bbPress.org

Sorry for the confusion.

It’s a WordPress plugin. And no, you don’t need to install any avatar-upload thing, or indeed any other extra plugin to get this to work.

The function works both on WordPress and bbPress, but since it is a WordPress plugin (and thus a WordPress function) you will need to make WordPress functions available to bbPress. This is covered in the Integration with WordPress page in the bbPress documentation (see the last section, “Functions”).

I hope this clears things up a little.