Mystery image in an MKV file

[PeterProg]

I've downloaded a few documentaries in mkv format from a file sharing site - all good, enjoyed watching them. But there is a thumbnail displaying with some of the files that I would like to change. Should be straightforward, but...

When I load up the file in its present form into mkvtoolnix, there is the video file, audio file, a subtitle file. Nothing more - no thumbnail, cover, tag - anything. Nothing in Attachments either. Likewise in mkvextract and mkvcleaver. Yet there is most definitely an image displaying with the file. Needless to say, re-naming the image I want to use to cover.jpg, loading into Attachments and multiplexing does not shift the original image.

Any thoughts on what this original image is and how it might be identified, isolated and removed?

Cheers

[ProWo]

Could be part of the videostream.