I haven’t uploaded decals at all so take it with a grain of salt. Your work looks interesting!
As you are talking about .png files… I think you can’t simply upload bunch of pngs. You have to either upload 3d project or Unreal project or Unity project (but Unity one has to be first accepted on Unity Asset Store).
IMO the best approach here would be to do Unreal project. I would advise putting them somewhere in materials category. See here about generic requirements: Asset File Format and Structure Requirements in Fab | Fab Documentation | Epic Developer Community .But I would advise reading the whole page from the beginning. Especially see the link to technical requirements (it describe project structure etc. https://support.fab.com/s/article/FAB-TECHNICAL-REQUIREMENTS )