Image attachment is a message_type = message with unique conversation_id

I can see also that for new images a "message.publish" action gets recorded, but I haven't yet gotten much further in investigating how the relation between these media and text content is being established.

Just wanted to leave you with a link to the Bulk Data API documentation which might also help you further this investigation: https://developer.khoros.com/khoroscommunitydevdocs/reference/bulk-data-api#export-reference