Any of the photo sharing sites run scanning filters that essentially search for skin tones and certain colrs/paterns, and note how much of a picture contains them. They are amazingly effective at flagging nudes and *advertiser censored* for review and possible deletion. It is part of the mechanism they run to prevent their sites from being distribution nodes for child *advertiser censored* or similar things. So even keeping the pictures private may get them flagged and deleted. not saying they catch it all, but there are mechanisms in place.