First, let me say I think this semantic discussion of whether it’s “theft” is somewhat distracting from the central issue, which is that accessing someone’s private photos and distributing them without their permission is wrong. And when it’s a case of revenge porn or whatever one wants to call it, it’s vile and should be considered as you suggested; as sexual harassment, a violation of a person’s rights, and a crime.
To the issue of whether it’s “theft” or not, I think you’re thinking of it too narrowly - it’s not the property that’s being stolen, since as you point out, in the digital world the original remains in the possession of the owner even when the file has been copied. But what has been stolen is the person’s right to determine when and how that picture is distributed. It’s not depriving the owner of the property, but of the use of that property that makes it a theft.
