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Ok nice. Yeah I uploaded some drone footage to d tube the other day and the thumbnail was like 30% of the original size.. so the key is to upload and then add in the thumbnail link back in Steemit as an edit?

Yeah but it won't still make it clearer on Dtube. Only for the steemit post.
I'm pretty sure Dtube will fix it next update, a lot of people have been complaining about it