Install mplayerthumbs
sudo apt-get install mplayerthumbs
Now, open Dolphin settings:
View modes>General>Previews>[x]Video files (mplayer thumbs)
You may need to add the following line to apt sources:
deb http://ubuntu.mirror.cambrium.nl/ubuntu/ precise main multiverse
Now update sources and then install again. If it still doesnt work, you can download a deb from here.