It's pretty straightforward and I personally hate the fact that these things need to be spelled out but whatever, it's a fairly subjective thing so it could be useful to get some clarification I guess.
basically to be safe: keep it safe for work/pg rated.
Before you post an image ask yourself:
"If this came up on my screen with my boss/grandma/little brother etc looking over my shoulder would a reasonable person following reasonable community standards be worried?"
If the answer is yes then you've probably overstepped and are in territory that can get you into trouble. There'll be some differences mod to mod with how they judge the situation and whatnot but if you follow that you'll be absolutely fine.