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!