Put a 122x28 size picture named Netherlands.png into the "res_mods\gui\mod_ship_icons\nation_flag\1" folder.
(My temporary solution attached)
Ingame you need to configure the V2 option to see the image.
In "res_mods\gui\mod_ship_icons\nation_flag\2" folder you need a more straight flag in the same name and size to use. That will be the V3 option in the configuration.
Good experimenting fellas and please share your ideas and results.