Testt/training modes are presumably needed?
Surely, a scanner that didn't have the capability of saving images for training (or testing) purposes would be pretty crap, unless it was thought worthwhile buying different scanners to do the testing on (even if they were basically the same scanner with different firmware).
In the end you have to trust the people involved not to [be stupid enough to] capture images when they shouldn't be doing that, and then disseminating them, and/or trust the manufacturers to set functionality such that in normal usage, images are always transient.
Stepping back, however, and thinking more generally about a surveillance society, unless an image is identifiable as a person, or comes labelled with a name, there's no obvious damage that could result to the subject even if the image was posted on the internet - if the person involved never found out, nor did anyone who knew them, they couldn't be said to be being harmed in any obvious manner.
I guess if someone was paranoid, the worry it might be happening might be harmful to them, but they could worry whether or not anything was actually happening at all, let alone happening to them.
It's also hard to see there'd be much of a market for body scan pictures, unless they were identifiable (which would put people sourcing them at real risk of being found out).
People who want to look at naked bodies on the internet aren't short of choice, nor are people who want to look at more-or-less clothed bodies and use some imagination as to what's underneath.
For people who get a kick out of real or staged candid pictures, I'm sure they can already find enough.
Maybe there'd be a niche fetish market for body scan pictures, but I'm not sure it'd be one worth someone risking their job and/or a conviction in order to satisfy.