Alas. The player usually determines the thumbnail.
Ah, there is no way to embed a jpeg and assign that as the first frame to be viewed?
I remember that some dvd maker program I had years ago had that feature, you could either select a frame form the video to be the preview frame, or add a picture file.
Assumed one of the adobe apps would have this feature.
Some video formats (e.g. Quicktime) do allow a poster frame to be defined within the metadata, but PP/AME doesn't support it.
Adobe's dvd maker program Encore allows you to choose a poster frame for their video buttons that display in your Encore menus
maybe that is what you are thinking of?
and yes, I would like to choose the custom thumbnail for video as you too