Digital images are not your stuff.
Digitization doesn't really have anything to do with it.
Exactly the same issues would be involved if the guy had pinned a Polaroid on a public corkboard, rather than posting a digital pic on a website.
Once somebody has an image of you it's theirs to do with as they please.
This is one of those very rare instances where I disagree with you.
There can be (explicit or implied) limitations as to what you may do with what you have been given (see below).
You're missing that these women GAVE AWAY their privacy.
Insofar as these women "gave away their privacy," they gave it away ONLY to specific persons, NOT to the general public.
If you don't want someone else to POSSESS a naked picture of you... then don't give it to them.
The issue here is not the mere possession by someone else of a "naked picture of you" which you have given to that person.
The issue is whether there exists any limitations upon what one may subsequently do with what one has come to possess.
Pro tip: if you intend to give up POSSESSION of something while still retaining OWNERSHIP....
SIGN A CONTRACT
As you have stated it, this is inherently contradictory - contract or no contract, one cannot simultaneously "give up possession" of something
and "retain ownership" of that something. One may, however,
conditionally give up possession (and therefore, also give up ownership) of something - and the condition(s) involved may be explicit or implicit.
As an example of an explicit condition, I might give you my car under the explicitly agreed-upon understanding that you may not sell it unless you offer to give it back to me first. If you agreed to this condition, the car would be yours, and you would have all the responsiblities and privileges of such ownership and possession - with the exception that you could not rightfully sell it to someone else unless I first declined to resume possession of it.
As an example of an implicit condition (or implied contract), when you enter a restaurant and order a meal, you do so with the unstated understanding that you will pay for the meal after you have eaten it. There is no need to sign any contract or make any other kind of openly-stated or explicit agreement - and anyone who tries to score free meals by dining at restaurants and then refusing to pay (because no contract was signed or no explicit agreement was made) is a mealy-mouthed scoundrel.
Just as with the implicit condition (or implied contract) involved in restaurant dining, it is perfectly reasonable to expect that if your "significant other" gives you (or allows you to take) "naked pictures" of him or her, he or she does so under the implicit understanding that such pictures are solely for your personal use/viewing/enjoyment/etc. and that (absent any explicit permission to the contrary) such pictures are not to be made available for the use/viewing/enjoyment/etc. of any other person or persons (let alone the general public).
In fact, this expectation is so reasonable that it seems rather absurd to assert that a signed contract (or any other kind of explicit agreement) is needed or required in order to spell it out.
IOW: Unless it can be shown that the reasonable presumption of "exclusive use" as an implicit condition of ownership/possession of these "naked pictures" was not in effect, the ex-boyfriend in the OP is in the wrong and he can rightfully be held liable ...