Oof.

They need to implement content addressing for "sidecar" signature files (add a hash) both to prevent malleability and to allow independent caches to serve up the metadata for images of interest.

Also, the whole certificate chain and root of trust issues are still there and completely unaddressed. They really should add various recommendations for default use like not trusting anything by default, only showing a signature exists but treating it unvalidated until the keypair owner has been verified. Accepting a signature just because a CA is involved is terrible, and that being a terrible idea is exactly the whole reason who web browsers dropped support for displaying extended validation certificate metadata (because that extra validation by CAs was still not enough).

And signature verification should be mandatory for every piece, dropping old signatures should not be allowed and metadata which isn't correctly signed shouldn't be displayed. There's even schemes for compressing multiple signatures into one smaller signature blob so you can do this while saving space!

And one last detail, they really should use timestamping via "transparency logs" when publishing photos like this to support the provenance claims. When trusted sources uses timestamping line this before publication then it helps verifying "earliest seen" claims.