Just to jump in here...
I think a lot can be read from the tone of the person. I have people on FB where they might show something like that and it would be just to update their status - losing weight, getting fitter, new haircut, etc. I totally take it as posted as I know that person and I could see it was genuine.
Then there are people who you can see a different tone. You can't really pinpoint it, but something is "off". if that person were posting frequent shots of their body... I might read it differently.
And maybe that's my own insecurities who knows.
With that said, at 44, I can hardly imagine ANYONE on my FB feed posting photos of themselves in their underwear only - period. I would think they were hacked!