Keep in mind it is a public forum that anyone can view. There is no disclaimer saying that there are explicit or partially nude images on the FFB so it has to be kept as clean as possible.
We frowned upon pictures that were linked from pornographic websites, showed full nudity/pubic hair, or ones that were simply just going too far.
There is no defined criteria; but surely common sense can prevail as to what can and cannot be posted.
Some things slide while others are picked up on because they cross the boundary. This is not just pictures, but comments that may go with or after them. I have deleted countless things that border on extremely inappropriate. If I had my way this thread would be gone, but some people understand the rules and others do not.
You are told if it goes over the line- but continous breaches of it will result in the thread being moderated harsher or even deleted. It is lucky to be here as it is.
In my eyes, there is no difference between a naked photo showing a woman's behind and a photo of her wearing a g string in such a manner. When there are complaints, we act.
"They shouldn't open the thread then"- This is not in the spirit of the forums as they should not have to wonder if they are going to be offended. That is what our job is.
Hope this helps.