Regarding OP’s question - are you referring to Video Preview Thumbnails? Sometimes people use that term when they’re really asking about Posters. 
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VPT thumbnails are extracted on a time interval, but (for extraction performance) only the encoded video’s keyframes are considered.
Some of the ffmpeg options used are mentioned here:
Video Preview Thumbnail Generation Fails for Some Items - #15 by Volts
If you felt crazy, it would be possible to manually extract the images, edit and customize them, generate the .bif file, and store it.
