Adding video to slides

Cédric, I have taken enough of your time, for which I am very grateful, but before ending will just summarise what I have found.
The full frame of black at the start of the video is not on the clip itself, but appears to be caused by the delay in buffering the video from youtube. I don't know if this could be shortened if the clip was stored locally on the hosting server, like the photos are.
The dummy photo for the video which I use to trigger the slide show appears to show both before and after te video clip.
The following has one photo, then one video clip, with it's dummy photo (which is a screen shot of the first frame of the video).
If you let me know when you have viewed it, I will remove it from YouTube.
So for now, I will stick to just photos, for which your tool does an excellent job.
Thanks,
Steve Marsh