The examples of media uploads I found in my data so far are actually avatar uploads, so they are not tied to a message id. They will all land in the user's media album though which is what explains why they are registered as a "publish" event. I agree though that it is confusing that it's a "message.publish" event and not a "media.publish" event.
After further digging into the depths of the Bulk Data API it looks to me like there is indeed no event triggered that would help you establish the relationship between media and - if and where - it is being used within a conversation topic.
The message.publish events you found for media uploads seem to be triggered either during the avatar upload flow or on the post message page (e.g. while the message editor is opened and the "upload media" feature of the rich text editor is being used). But there is no event listed on bulk data event list that ties the media on post creation to the actual message.
An alternative to obtain which message(s) an image is being used in would be to use API v2 and a LiQL query like:
SELECT id FROM images WHERE messages.id ='187'
