I agree that it is going to be very subjective and there is no way one can define in this kind of forum where there is a wide spectrum of members from all walks of life with different levels of 'maturity'.
What I think is more important is being sensitive to the people in the forum and place pics with a link if you think that it may be offensive to some.
If you find half the ppl here slamming you, I'm sure you will know that the pic is offensive.
There have been some threads with the pic showing some undergarment of a model sitting down and this has not gone down with some though some say that it is ok.
i personally find nude model shots slightly offensive
i remember someone posted "indie shots" of a transgendered subject and the response was quite contentious.