Install mplayerthumbs for video thumbnail preview in Dolphin

sudo add-apt-repository ppa:kubuntu-ppa/backports
sudo apt-get update
sudo apt-get install mplayerthumbs

Ref: http://pkgs.org/ubuntu-13.04/kubuntu-backports-i386/mplayerthumbs_4.11.0-0ubuntu1~ubuntu13.04~ppa1_i386.deb.html

Now enable thumbnails in Dolphin:

Dolphin>Settings>Configure Dolphin>General>Previews>
Enable “Video files(Mplayerthumbs)