Right, because the timeline image only exists as a preview image inside PD, while all media library clips are true external entities. PD requires "produced" (actual) clips, for importing into the Motion Tracker.
You'll just need to do another round trip using GIMP or Paint.net on a timeline snapshot, as originally described in this
thread. Once that transparent image has been created, import it into the media library and then into the Motion Tracker.
YouTube/optodata
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