Actually I think you'll find naked bodies a whole lot harder to identify than the clothed bodies...
I think taking photos of people without them being aware of it is highly unacceptable. I think you'll also find that if photos are taken in public places for the intent of promulgating to the wider population, consent must be sought for that. If you attend conferences where photos are taken, consent must be sought for those images to be taken (even if they're aren't used).
I think that one should reasonably be able to enjoy a flight (especially a long haul flight) without fear that another passenger is going to take photos of them... I'd be very unhappy if someone took a photo of me during a flight, even if I am committing some horrendous crime like putting my feet up on the seats/bulkhead.
I'll also wager that the OP would be more than happy to lie across multiple seats if available, with his feet touching the seat, if that meant he could sleep. Is that a travesty?
The point I'm trying to make is that sure, the OP was offended, but there's a continuum of offenses and I don't think this rates very highly in terms of serious harm to others.
Stop invading others privacy with photos and ask them nicely to desist from whatever it is that's bothering you.