Kde, Dolphin : Dolphin not showing preview of images or any files
I had exactly the same issue. I used
sudo apt-get install kio-extras
and now it works well. You might need to go to
Settings>Configure Dolphin...>General>Previews
and then select whatever you need. Then of course press the "Previews" button if you haven't or go to View>Previews, but at least images should be by default. I also read that for some other formats (that I'm not using, so I haven't tested it) you should try
sudo apt-get install ffmpegthumbs
sudo apt-get install mplayerthumbs
sudo apt-get install kffmpegthumbnailer
I hope it works for you too!