[2021] MOVED: Import Transparent PNG's as Decals

For decals you wouldn't make the diffuse texture transparent, but you add another texture called alpha, which is a greyscale texture that tells the game what should be transparent. Black is fully transparent, white is completely solid.

Here are a few links related to asset creation and specifically decals:

https://skylines.paradoxwikis.com/Asset_Editor#Modeling
https://cslmodding.info/shader/decal/