ok, i always wondered how to get these bacause everybody was asking for them including myself so i started researching and found that in the gconf there is nothing set to the videos to get thumbnails only images... so this is what i did..............
1- start yumex (or your package manager prefered)
2-remove totem and install totem-xine (if havent already done)
3-install gstreamer-plugins and gstreamer-ffmpeg (i've got installed everyone of them)
5-ok, now open gconf-editor, i did this as normal user
6-look for the folder /schemas/desktop/gnome/thumbnailers and add the following
7-click on the key you want to modify for example video@x-msvideo and right click on the right panel ->New Key
Value: /usr/bin/totem-video-thumbnailer -s %s %u %o
Note: you have to add those lines for every Key you want to modify mostly video ones, after that just close gconf-editor normally.