ApichatWebhookImageMessage

timestamp
Type: number
required

Unix epoch seconds with fractional microseconds (e.g. 1780054017.039184). Not a string, not milliseconds.
type
enum
const:
image
required
url
Type: stringFormat: uri
required

Publicly-fetchable GCS URL (…//channels/.).
message
Type: string | null

Caption; null when none. NOTE: multiple images in one block fire multiple image POSTs ~30ms apart — group by customer.token + time window.