Thumbnail poster frame in footage problems

I am having problems with Adobe Stock with some clips where the automatic thumbnail Adobe creates is totally misleading. For example if I make a slide shot from white background to the subject the thumbnail is white......how is a buyer be interested in such a file?. Adobe is the only footage agency where it is impossible to change the thumbnail/poster frame. I try to do this with stock submitter and davinci resolve but even so many times Adobe Stock just picks up the first frame. It is really frustrating. Someone with a solution about this issue.

Thank you

Cristian