RetroArch should pick generic icons if playlist name does not match any icon name from assets, otherwise you'll get the black squares all over the interface.
I like to make custom playlists, for example:
Arcade - Capcom - CPS-1
Arcade - Capcom - CPS-2
Arcade - SNK - Neo Geo
See:

It should be nice if RA pick "Libretro - Pad.png" for the platform and maybe "file.png" or other icon for content. Example:

+1
This seems to have been implemented in the master. I removed the asset images for one of the playlists and it showed me an generic icon instead. Additionally it will no longer share black boxes if the assets are missing.
I think its safe to close this.
Could anyone elaborate on this? Are default icons just mapped 'automagically' in the latest build?
If you add a new playlist manually, which is not recognized by RetroaArch (not having own icon/assets), it will by default add a playlist icon and content icon with default.png asset (currently rocket icon): https://raw.githubusercontent.com/libretro/retroarch-assets/master/ozone/png/icons/default.png
Most helpful comment
+1