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Today i can give the answer by myself !

YES you can edit/change the Video thumbnail as following

  • Go to D.Tube and press the UPLOAD button
  • Upload your new or edited Thumbnail (snapshot)
  • Scroll down and copy the Hash for Snapshot
  • Go to your Video where you want to edit the thumbnail
  • Press EDIT
  • insert (paste) the new generated and copied HASH to the Snapshot Hash field
  • Press Enter

And here it is!! your new thumbnail for an old video

Badumm tsss!