according to the community rules here, you avatar must not be of "crass or sexual nature" and it is up to the moderators to decide what is or isn't. so i guess its up to the mods interpretation of the nudity, but several members do have bare breasts in their avatars
Personally, I'm uncomfortable seeing nudity in avatars, especially because I occasionally show my parents certain posts here. I don't think they'd enjoy seeing nudity on a website I peruse, and neither do I.
Then again, I'm not a moderator and it's not up to me to decide.