Dolphin does not show thumbnails

Thumbnailer

To show thumbnails the Dolphin will need the thumbnail plug in.

Video thumbnailers for the Dolphin:

:~$ apt-cache show ffmpegthumbs
Package: ffmpegthumbs
Priority: optional
Section: universe/kde
...
Description: video thumbnail generator using ffmpeg
 FFMpegThumbs is a video thumbnail generator for KDE file managers
 like Dolphin and Konqueror.  It enables them to show preview images
 of video files using FFMpeg.
 .
 This package is part of the KDE multimedia module.

Homepage: http://www.kde.org/

.

:~$ apt-cache show mplayerthumbs
Package: mplayerthumbs
Priority: optional
Section: universe/kde
...
Description: video thumbnail generator using mplayer
 MPlayerThumbs is a video thumbnail generator for KDE file managers
 like Dolphin and Konqueror.  It enables them to show preview images
 of video files using MPlayer.
 .
 This package is part of the KDE multimedia module.

Homepage: http://www.kde.org/

.

:~$ apt-cache show kffmpegthumbnailer
Package: kffmpegthumbnailer
Priority: extra
Section: universe/kde
...
Description: video thumbnailer for KDE
 kffmpegthumbnailer is a video thumbnailer based on ffmpegthumbnailer.
 The thumbnailer uses ffmpeg to decode frames from the video files.

Homepage: http://www.kde-apps.org/content/show.php/kffmpegthumbnailer?content=117562

Settings

To use the thumbnail you need to enable the thumbnailer from the Dolphin settings.

In a terminal run

sudo apt install ffmpegthumbs mplayerthumbs kffmpegthumbnailer

then... change in dolphin settings as says in user26687's answer... works for me kubuntu 14.04lts in laptop n4010 dell

Now in Ubuntu 15.10 I needed another package for thumbnails to show:

sudo apt-get install kio-extras