Even as I'm working with Kdenlive for more than two years now on 40+ tiny projects, I still lack any clue about some mysteries. In particular: what is the checkbox "transparent background" (sorry, don't have the exact description at hand as I'm using a localized Kdenlive version) in the clip properties dialog good for? I've tried to play around with it but never found out how it works. The current Kdenlive documentation also seems to fail on this setting ... so, one of Kdenlive's better kept mysteries?

Yes. I wrote that bit of the manual.
http://userbase.kde.org/Kdenlive/Manual ... _Images.29
"The Transparent background for images checkbox labeled 4 - is for "
which is not real usefull. I would like to know what that does too.

Just stumbled on this http://opensource.com/life/11/11/effects-and-transitions-kdenlive

"
Composited images and titles

If the material you are importing for compositing is an image or an image sequence, such as an export of sequential png's from Blender or Synfig Studio, then they probably already have an alpha channel. In this case, all you need to do is use the Add Clip option to add the sequence to your Project Tree.

As long as you have checked the boxes at the bottom of the file chooser dialogue box ("Image Sequence" and "Transparent Background Images") then the images will be imported as a single video clip with built-in alpha, meaning no color selection or chroma key is required. Simply place the image sequence over a video clip, add the Composite transition, and move on."

So I guess the "Transparent Background Images" makes it take notice of the alpha channel when importing a sequence of images and generating a clip from it.