The answer can be found in Section J of this post
Generally, the guideline is "be tasteful", but with such a wide variety of people as are on Elliquiy, that guideline's borders are gray and fuzzy at their best.
No avatars that have the appearance or suggestion of condoning illegal activities. No lower genitalia should show. Breasts are fine most of the time. Try to keep it appropriate for a teenage audience. Avoid crude images.
If you ever have a question about what is and is not okay, feel free to PM a staff member and ask.