I don't think there's anything to do about it, really.

This allows for the maximum amount of visual presentation, while preserving space to allow for as many thumbs for each torrent as possible.

On the other hand, the type of thumbnails that show multiple snapshots from within a single movie convey much more readily the full contents of a particular scene without the need of a long text description.

Here's a typical thumbnail I'd upload for each file contained in a torrent:

It's created with Video Thumbnail Maker. A free version is available, or it's only about £20 (if I recall correctly) to get a licence. I see many people here use the free version, which embeds the software's logo and info in one of the screenshots. What many people don't seem to be aware of, or at least utilise, is the ability to have multiple sized frames within the same image. This means you can show both a couple of reasonably sized shots with the performers more clearly visible, and the 'story arc' of the whole scene. If there are only a handful of files, thumbnails don't need to be included in the torrent but can be added as part of the torrent's description. Indeed, I'd argue this is a better way to do it because it makes seeding / reseeding much easier without forcing a downloader to also keep unwanted images.

For larger torrents, including the images makes sense. They can be downloaded first and then be used to judge which particular files you might want. I would, though, add a plea for these images to be in a separate folder to the main files. If a torrent contains fifty movie files with a thumbnail for each, that's great, but if you have to go through the file individually selecting or deselecting just to get an initial thumbs only download to select from.

I'm sure it's hardly worth mentioning that when you upload a torrent, the rules do stipulate "GOOD QUALITY" images. I don't personally feel that images that are 300px by 150px, for example, meet that requirement.