I didn't know it five minutes ago, but I just checked.

It loads the decal data from an IFL (image file list) which is only updated on startup. I checked and it's something to do with the function AvatarGUI_CreatePartMenu();.
Not that this helps, that's a client side function and you'd need client commands for it to work for people joining your server.
If they are willing to download a script to do that, why not just download the decals?