Having NNF display avatars by requesting an image with the same name as the user is very possible. Although I would advice using some sort of alpha-numerical digest of the user name, because NNF supports a lot of different symbols in the name that might upset your server OS.

The problem is allowing people to upload these images. NNF does not have any sort of control panel for such an action so you will have to invent one. That pretty much means starting from scratch on a control panel.