Enable video thumbnails in Dolphin

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.