You won't go far wrong following bear1515's advice.
When you post a torrent, the instructions say you must upload at least one picture. This is often a cover shot of the movie. Screenshots are not required but are very helpful. Some people say they won't download without screenshots but that's their decision.
For thumbnails I use Media Player Classic as suggested. I use 6 columns by 6 rows and this usually gives people a good indication of what to expect if they download the movie. Others may choose other dimensions.
Even if you don't post screenshots, once anyone has downloaded the complete movie there should be no reason they can't make a screenshot and upload that (they get a bonus point) - sadly this doesn't seem to happen. But there is nothing to stop you asking, in your description text, a downloader to make them once they have the movie.
For me, if I have a movie on my hard disks and there are no screenshots with the torrent I usually add them. I would certainly be happy to do that with yours. As all the movies are posted anonymously, feel free to PM me if I can help.
Happy posting (smile)