The line of NSFW is nebulous by nature, but in general it's exactly what it implies "if my boss walked up behind me and looked at my screen would I be okay with them seeing what I'm looking at?" That means full nudity is not the only thing that might be considered NSFW, but it of course depends the individual. When in doubt, it's usually safer to tag it since it's not typically going to deter anybody except children from clicking on it in any case.