FFmpeg allows phpWebSite to create thumbnails for their flash video
files (among other things slated for the future).
Unfortunately, just installing FFmpeg won't necessarily help.

FFmpeg does not ship with flv (i.e. Flash video format) compatibility
by default. In order to enable it, you will need compile it
separately.

If you want to test your installation of ffmpeg, try the following:

ffmpeg -i movie.flv -an -s 340x240 -r 1 -vframes 1 -y -f mjpeg /
movie.jpg

This will take the first frame of a movie.flv and create a jpeg
file. If it doesn't work, then it will not work in phpWebSite.

Also, when setting the ffmpeg directory in file cabinet, enter the
directory it is "in" not the command itself:

$ whereis ffmpeg
ffmpeg: /usr/local/bin/ffmpeg

So you would enter:
/usr/local/bin/

NOT

/usr/local/bin/ffmpeg