I don't know what the policy is on other SE sites, but if there's a network-wide policy on graphic images we should follow that. My answer is basically the same as Eric's comment, but I made an answer so that it could be voted on.
I don't think we should be worried about graphic violence in images posted here. The only people who will read the question and answer linked to are users here or people who google for something related to Highschool of the Dead. In the former case, SE requires all users to be at least 13, and generally a user here should know that HOTD is a fairly graphic series if they intend to click on the question. In the latter case, anyone searching HOTD is sure to have seen plenty of graphic content related to the series, so we shouldn't be worried about that either. If you have especially graphic or disturbing images, a spoiler tag could be used with a warning, but I don't think we should require it in general.
Nudity should not be allowed, mostly because it's unprofessional and usually unnecessary. Some people may check this site from work computers, so any NSFW sexual content should not be shown. SE has a policy banning sexually explicit and pornographic material, which of course is a bit ambiguous. I'd lean on the safe side here and say that anything questionable should not be directly posted here.
The SE policy above explicitly includes links to sexually explicit content or pornography. I personally have no problem with people linking to NSFW content with an appropriate warning if it is absolutely necessary to ask/answer the question (and if it's an acceptable question). But until an SE employee clarifies this, even if you are not violating our site's policy, you may be violating SE global policy. I doubt you'd be banned for that if you are legitimately using the site properly, but I don't know for sure so tread with caution.