How to add them... it is simple to fo, but there is no simple answer because you can do it different ways for different censorious. One, you can add this thru presentation/placeholder settings as 'use image' for background settings. or, second, you can use Image Widget for that. Videos can be added only (sort of) using widgets... most likely Video Widget, but HTML-crafty folks can use HTML or Web Widgets, OR switch to HTML view and do their magic in there. Pick your poison.