Thanks to Silvan  who mentioned that feature of mplayer. Quite easy that way, to get certain images out of a movie. mplayer is available for several operating systems .
mplayer -frames 1 -vo jpeg outdir=/tmp:quality=75 path/to/movie.mov
Who is going to make mplayer a PHP PECL extension? 😉
Update: Some nice tips by Ben Edgington  using mplayer, imagemagick, mplex etc.