>>100537286
ffmpeg -i input.webm -vf "select='eq(n\,0)+eq(n\,50)+eq(n\,100)+eq(n\,150)',scale=320:-1,tile=2x2" -frames:v 1 thumbnail.jpg
realistically though what you want is to make sure your webms are encoded in a manner that windows can parse a thumbnail from the video file itself automatically. ffmpeg should be able to do that just fine.
i know mp4s have moov atoms and shit for this, webm should be not much different. consult chatgpt or something for getting a script for reencoding if your webms dont have thumbnails automatically