If you happened to find naughty/nude pics of someone you know or are 99% sure it's them would you tell them you saw them? Would you ask if it was them or say something like "do you put pics on x"?
I wouldn't. I wouldn't want to make them uncomfortable. I have done stupid things on a whim. Maybe they did, too. I wouldn't hold it over their head. Maybe it was just a stupid decision he/she made that he/she now regrets.
I wouldn't say anything for the reasons @Ashley01 mentioned. It's possible that they might not even want others to know or have knowledge of it. Similarly, if say a friend or somebody I know would approach me about a situation such as this and feels guilt or bad about it, and if I'd seen the picture beforehand, I still wouldn't say something. I would want them to continue feeling comfortable and at ease, and not worrying whether others have come to know about it as well.
On the other hand, they might not even know there is a picture of them. Maybe it was posted without their knowledge and that is an issue in itself.
