Jörg, I'm pretty sure that option does nothing (at least with video). I see some code in the HUD to get images via Mavlink and display/save them. But nothing about video except in the name of that menu item. I should probably get rid of that menu altogether... it's one of the remnants from the original QGC, which had quite a number of unfinished or unusable "features" when I started with it. Still finding some new ones 3 years later...
From a quick look in the code, I think the intention (at some point) was to have a separate widget for video... the class is called and I see some code in there that Peter had added to display video with VLC. The code is still included but the video widgets are commented out in . I don't know if Peter got anywhere further with video in QGC.
Safe to say that right now there's no video display capability in QGC until someone adds/fixes it. Perhaps they came up with something useful at the original QGC project which could be brought over. I personally don't have any plans or time to work on this, but I'd certainly be open to some well-implemented solution that anyone comes up with.
-Max