Hi!
After I manually installed multimedia codecs, like w32codecall, I got Linux to play movies and I also got thumbnail previews of them in Dolphin. However, I noticed that recently Dolphin doesn’t display my thumbnails anymore. I don’t quite recall whether the any video player could use my installed codecs, at least xine works. The only thing that could have caused that is actually the automatic system updates I received. I still have my multimedia codecs installed, but Dolphin doesn’t show the movie previews anymore.
Did you try re-checking the ‘Preview’ icon in the menu?
If forgot to mention, yes of course.
Check and see you didn’t install a package called mplayer-thumbs
It’s not needed. it’s from Packman I think.
Other than that we can delete your dolphin config files and see if we start again!?
How does Dolphin use the thumbnail function by the way? Couldn’t it be the case that Dolphin and other movie players don’t find the codecs anymore? I remember how there are a couple of different possible codec folders, and perhaps the system update messed something up.
Try this in a terminal (make sure dolphin and konqueror are closed)
mv .kde4/share/config/dolphinrc .kde4/share/config/dolphinrc-bak
logout immediately and try reconfiguring dolphin
If you want to go back to your original it’s still there:)
I noticed that I don’t have mplayerthumbs (anymore?) for some reason. For another reason, I don’t find it in my repositories, although I have the SUSE repositories and a Packman repository.
Strangely enough, I don’t find any Edit function here. I noticed that the preview works fine in Gwenview, don’t know how Gwenview does this.
But did you try rename of the dolphinrc file as suggested?
Yes, I did this and it didn’t change anything to the better.
Here is what you need to do:
In YaST, install:
kdemultimedia4-thumbnailers
kffmpegthumbnailer
libffmpegthumbnailer
ffmpegthumbnailer
Once that is done, go to Dolphin->Configure Dolphin.
Click on General->Previews.
Scroll down to Video Files (Should be 3 of them), and check them all.
Previews should be working now. 
I installed kffmpegthumbnailer (opensuse 12.1) updated the dolphin settings for video previews and that worked for me. hope that helps.
Old thread, but just want to say this also work on opensuse 13.1.