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I'm not sure but I think you should edit and place (upload) the thumbnail you want and finally update

Dtube used to have options for editing stuff like that. Here in Busy or other places it does show the correct thumbnail, but in dtube it does show wrong one, as bit of a perfectionist this is quite annoying. I try to contact Dtube staff about it.

I have had some problems with that platform. I hope you can solve