It would seem to me that trademark of your own image is inherent in personhood - that someone must have permission to use that image in any way.

Which is, of course, the whole point: people are giving the TSA the right to use the images for that specific purpose, and we have no idea what other purposes they might be used for after that. And that's not even counting my discomfort with having my naked body viewed by someone in a security room somewhere for the stated purpose.