Edit^2: On triple reread it sounds like the first pass ("Image Capture") sends the image metadata hash to be timestamped, whereas the second pass (Reference Image Development) sends the image itself but is not what actually creates the timestamp attestation. According to Apple[0][1] it sounds like the second pass (development) only happens when the reference image is actually viewed, which means that your image isn't sent if you never view the reference image?

[0] https://support.apple.com/guide/iphone/view-reference-images...

[1] https://www.apple.com/legal/privacy/data/en/reference-image/

> When you take a photo in Reference mode after tapping Reference Mode, your device will include reference image information in the photo’s metadata. If you then view that photo and tap the Reference badge on your iOS device or click it on your Mac, the device will send the raw photograph, metadata about the photograph like the sensor’s signatures and the time frame in which the photo was captured, as well as the sensor’s unique hardware identifiers to Private Cloud Compute.

Edit: On reread it seems they do in fact send the actual photographic data to PCC, which I presume has some reason over signing metadata on-device? Original mistaken post is below for transparency.

You can always not use the reference image mode, and according to the article you send a hash of the signature of the photograph, so all they would know is you took a photograph in reference image mode at some point in time before the request.