Change Thumbnail on video in iXpand

I bought the ixpand drive so that I can put movies for my kid on it for our long haul flights coming up.

Problem is she is 3 and can’t read. So how can I change the thumbnails to something she can recognise, like a still of the movie or a movie poster etc

I’ve searched all over and can’t seem to find anyway to do it.

Surely there must be a way other than the random one that pops up or in some cases we get nothing at all.