muOS

Box Art / Preview Art / Game Text is no longer tied to the directory structure you use for your ROMs.
All of these files are now stored in a location referred to as the catalogue.

Folders for all systems should be created for you.
These systems match the system names in core.json and are fixed.

All gamedata should be named to match your ROMs.
The folder named Folder can be used to apply artwork to any folder in content explorer and just needs to match the folder name
(not case sensitive).

SD1
└─ MUOS
    └── info
        └── catalogue
            ├── <System>
            │    ├── box
            │    │   └── romname.png
            │    ├── preview
            │    │   └── romname.png
            │    └── text
            │        └── romname.txt
            ├── Folder
            │    └── foldername.png
            └── Root
                └── box
                     ├── sd1.png
                     └── sd2.png

As muOS now assigns artwork by system, for it to know which system a folder contains you will need to assign a core before artwork is displayed.

Example
If you have some ROMs in:

SD2/roms/Nintendo/SNES/Action/*.zip

Catalogue files go in:

SD1/MUOS/info/catalogue/Nintendo SNES-SFC/

To have folder icons for all subdirectories listed, you would need:

catalogue
└── Folder
    ├── nintendo.png
    ├── snes.png
    └── action.png

Skraper mixes, and Artwork for the Tiny Best Set are available here:
https://github.com/antiKk/muOS-Artwork