Choosing white 'margins'

[Mara]

I do hope I have even the terminology correct but am having difficulty figuring out how to make the area that shows around the edge of each photo white, rather than black. (Pretty sure they are called 'margin' areas? - but just in case, thought I'd better explain more fully what I am trying to do).

 

Did go to the 'help' section and it reads, "The margin is the gap between the overlay and the edge of the video. Enter the margin size as a percentage of total video width (horizontal margin) or height (vertical margin), then press Ok.

 

My question is ... in what section is this done, please?

 

This is my first attempt to make a slideshow so, if possible, would truly appreciate simply, easy to follow directions - gentle smile.

 

Thanks so very much for any help...

Mara