I think there's a certain amount of exaggeration going on here. The OP has not gone to the police or anyone else saying 'underage photo'. (Even if she had, one topless photo sent by her is not going to result in a dawn raid. Particularly when they do the maths around her age now and the way the photo was taken by her last year.)
People do silly things when they're worried, and this was in that category, not worse.