Thanks a lot.

I did what you said and also followed the instructions from here. I then formatted the picture and changed the solid fill to black.

Now I am looking into putting in some videos onto the slides. I managed to find out how to play full screen and I will just change the solid fill to black as above. The problem is that some of the videos are upside down and the rotate option is greyed out (I use VLC player) so I don't know if there is a workaround for this (they are playing normally in VLC).

I am not sure what is a realistic file size limit for this whole project as I read that if the PowerPoint file is too big it might fail.