Most formats are accepted as far as i know but you'll have to try it out yourself.

dtube.video isnt the same platform as steemit it has simply copied the blockchain which is why it requires a private key to sign up.

The thumbnail will be whatever you set the video thumnail to be on the dtube.video website.

You can format the way you would like the steemit post to be layed out in the description box of your dtube video.