Earlier quoted context omitted.
How would the server know that the request is coming from a real iPhone? This is a pretty standard application of trusted computing and can be done entirely on the iPhone. A server would only possibly be needed for anonymization (while retaining key revocation capabilities if a key does end up leaking), but there are serverless ways to do even that (TPMs have supported these for a while now).
It wouldn't, but you could validate that a particular picture was created at a particular time, and had not changed, for example, especially with metadata that you may not want to share but that establishes certain parameters like gps coordinates. A lock, rather than an end-to-end pixel signature, which shows what was contemporaneous rather than exact provenance. If an event happened on day 0000-00-00 00:00:00am, but…
Nothing prevents anyone from opportunistically pre-generating and timestamping millions of permutations of fake kompromat and then selectively revealing the one that turns out to be useful after the fact.
You could charge per attestation, but the economics of that don't look great; you could demand publication of the image itself before attestation, but that would obviously not fly for most use cases out of privacy concerns.