I think there are two topics going on here: (1) I think the OP was asking about thumbnail files that would be part of of the torrent itself, and (2) others were addressing thumbnail files that would be part of the description on the torrent page.
Both approaches are useful, depending on the circumstances: If the torrent has only a few videos (four or less, maybe), then having thumbnail files posted as part of the torrent page is a good idea, along with some large single photos.
For torrents with a lot of video files, I like to see a separate folder of thumbnails in the torrent itself; then I can download only the thumbs and then decide which video files to download.
Whether a thumbnail file is part of the torrent page or downloadable from the torrent, advice from members above should be heeded:
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Set the thumbnail generator to make each thumb a good size--150 pixels high at a minimum.
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Don't have the video sampled too often--once every two minutes will give a sense of what's going on.