The key components you need are 1) a URL to an image and 2) a status message to go along with it (though this could probably be "" (null/a blank string) for a media-only post).
Screenshot → https://share.cleanshot.com/ktY34JSnbpGMwyTJQMjs
Assuming you follow along in my screenshot…
- Set a webhook as the beginning of the flow
- Copy that generated URL ("A" in the screenshot)