For anyone wondering how to install this. I spent some time figuring it out and will share the solution thusly.
IF you have Moreclothing mod installed (Like I do) - It governs not only clothing, but ALSO backgrounds, foregrounds and overlays. In the mod folder (in my case in the INIT folder) there are respective folders for background etc. They contain image files. Some of them might already work (in my case the jailbars as foreground)
What you have to do: Get the image, like this paperdoor png. Put the image file in the respective folder inside the Moreclothing mod folder, for paperdoor it would be foreground.
THEN - find the "moreclothingV16settings.txt" (in my case) file in the SETTINGS folder, NOT mods folder and find in that file the lines that list the backgrounds, foregrounds, overlays. ADD your own line following the existing example, it names the file name, gives it a name to be used in the loader. DONE
TL;DR
Put file in Moreclothing mod background/foreground folder, add line to use that image in the Moreclothing txt file in the Setting folder.
Simple, but it took me 3 hours to figure this all out, since there are not a lot of help posts online.
Good luck out there, may the gags sound lusciously