SWF Tools, movie thumbnail?

swftools does not handle such a function. You are looking for something that interacts with FFMPEG. investigate the flashvideo.module. FFMPEG binaries must be compiled on your server to use FFMPEG.

depends on how to prepare those thumbnails.

option 1:
u may upload a flv and also upload some thumbnails. show those thumbnails in teaser and show the flv in body.

option 2:
upload a flv and let drupal generate the thumbnails. show those thumbnails in teaser and show the flv in body.

depends on your skill set (do by yourself), or budget (out source), both ways are not very difficult but need some basic knowledge of drupal + video.

If what you want is a hand made still frame or image to accompany the video (and serve as the poster frame in the flash player, as well as used in teasers, listings made with Views, etc), then this is how I do it for my own site http://www.davidnewkerk.com/book/103#media
I do it this way because the videos are for my portfolio and I need to have the exact frame of my choosing (or even just a representative image such as the film's cover art, etc). In my case I also produce the .flv videos on my own computer before uploading them.

If you want something automated though, look into pairing Media Mover into the mix with SWF Tools, to process the video file (using FFMPEG) and produce a still frame, which you can then send into an Imagefield or elsewhere. Or there's also FlashVideo module, which may not be quite as flexible an approach, however it's likely easier to setup.