@baphomeat I found a way to find your wallpaper in settings. You want to change your background name in the persona.ini document. Let's say your webm file name is cats.webm . Initially, the Start Page template that erolok gave is:
[Start Page]
background = video.webm
position = center bottom
title text color = #FFFFFF
first frame image=poster.jpeg
We want to rewrite: background = cats.webm
It will look like this:
[Start Page]
background = cats.webm
position = center bottom
title text color = #FFFFFF
first frame image=poster.jpeg
If that does not work then it also might be your poster jpeg file name. You can rename your .jpg file into poster
Hope it works!