@willingmouth Video Thumbnail Maker is a free program that takes about 5-10 minutes to "learn" - or just ask for guidance here - then thirty seconds to load 500+ videos. It may take a while to do that many, but that's it. You can set it to output to a separate, specific folder, which you can then include in your torrent. Having the thumbs in a separate folder to the video files makes it far easier to download just that initially, make your selection, then download some or all video files, depending what the user wants. It takes merely a minute or two of extra effort, and is - to my way of thinking - both considerate to other members, and likely to improve how much gets downloaded.
Like @frostycab I have very rarely (not absolutely certain that I have never) left negative comments regarding thumbs within large torrents. I have, on occasion, uploaded those thumbs myself, when I've decided to download the full torrent and found them missing.
Images in a description is a different thing. "At least one good quality image" is a requirement, not a suggestion, and I see no reason why anyone meeting this very simple, basic,rule shouldn't be called out for it. Though relatively politely, of course.