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Good rule of thumb is that if it's suggestive (e.g. nudity, type of clothing, posture of subject, focus of picture, etc) it's fine in /e/. If it's explicit, i.e. focus of the picture is blatantly meant for sexual arousal and nothing more, then it belongs in /h/. This comes down to the "what is art" question, but it's a good way to get you to the right board in 95% of cases.
But when it doubt, post in /h/. Ultimately, what matters is what the mods decide.