You want to add a new avatar gallery but you don't know where is the right directory to put this new gallery. With Kunena forum, you can add some others avatar galley, you aren't limited.

For create a new avatar gallery, you need to create a new subdirectory in joomla/images/fbfiles/avatars/gallery/ , in this directory you can add your news avatars wich will be in the new avatar gallery .
Then when you have added a new avatar gallery, she will appears on your profile on my profile -> My avatar and then in the drop down list.