The original note, as written at the time
A WhatsApp feature that would involve the ability to show a particular image to particular people as your profile picture.
Ongoing development
A messaging feature that lets you show a particular image as your profile picture to particular people.
The profile picture is a broadcast, and the broadcast is the problem. You have one face for your mother, your employer, your friends from school and a client in another country, and the medium forces all four to see the same thing.
The workaround people use is a second account, which is a blunt instrument: two identities, two contact lists, and everything in the wrong one at least once a week.
Per-person images are the precise version. Not privacy — nobody is hiding anything from anyone — but the ordinary social fact that different relationships have different registers. The photograph you would send your sister is not the photograph you would send a bank.
The design constraint is that it must never be visible as a mechanism. The moment a person knows they are seeing a special image, the image becomes a message about them, and the feature has created exactly the awkwardness it was meant to remove. Silent variation is the only version that works.
Which raises the obvious objection: silent variation is also a tool for deception, and a system that lets a person present differently to different people is a system that lets them present falsely. That is true of every social medium ever built, and the answer is not uniformity, it is the ordinary expectation that people present the version of themselves appropriate to the room.
The interesting thing is that it makes the medium more like a room. In a room you do not show one face to everyone, and nobody thinks less of you for it.