Avatars and signatures need to be SFW. Faceclaims for characters don't have have to be SFW but they do need to be behind a spoiler tag noting its NSFW. Character face claims/Avatars/Signature images must also be at least 15 years of age, and look it.
EDIT ::
Heres a guideline from the rules on what can/can't be included in avatars/signatures ::
All Avatars & Signatures must be SFW. This means no genitalia (to include female nipples). Bare backside is fine so long as the cheeks are not spread such that the anus is revealed. Nothing tasteless or pornographic or it will be deemed NSFW. If staff deems it NSFW, it is NSFW. Any Avatar or Signature in violation will be immediately deleted.