What are deviantART's policies on nudity?

Nude submission are allowed, although certain themes are prohibited as obscene or pornographic (details are found elsewhere in the FAQ).

All images containing nudity must be tagged as Mature Content upon submission. The deviantART staff reserves the right to mark any work with a Mature Content if it is deemed by that staff member to require one.

We strongly recommend that photographers submitting photographs of nude models have the necessary model release form documentation recorded. While the model release form documentation is not mandatory as of this writing the deviantART administration reserves the right to demand such documentation at any time and to remove any work for which it is demanded but not provided.

Models under the legal age of eighteen (18) are not permitted to be depicted nude, partially nude, wearing undergarments (or lingerie), in any erotic pose or in any fetish-based situation.
The deviantART administration reserves the right to remove any such materials without notice and to place a ban on any account found to be submitting such materials without warning or notice.
This prohibition is not limited to photography and also extents to drawings, sketches, paintings, and any other form of artwork which depicts a minor or child in a similar situation or state of dress and carries similar consequences.
More information concerning this policy is available elsewhere in this FAQ.