It's the same rules that apply to the rest of the forum really, with a couple of extra guidelines for posting in projects :
If it's something that contains nudity or may be not safe to view at work, only post it as a link in the 'photos only' thread and make sure that you clearly state NSFW in the specific post in your own thread and structure it so that people have to scroll down to see the picture.
Extract from the general forum rules regarding nudity below - Note that for project threads, we've allowed members to include images in this section (i.e. outside of N&G) as long as they stick to the guidelines above.
Beyond that, there are no firm rules about subjects being off limits - just use a bit of common sense and don't post anything that the majority of people are likely to find offensive or in very bad taste (Not suggesting that you would BTW!)