How to apply a flag to an overriden extinct nation

Need a hand from those of you who are up to speed with new documents format.

I have overriden the nation of 'SEA Chinese' to create my fictional nation in the pre-game editor, however I cannot seem to be able to give the country a logo/flag and in game, the place where the logo/flag should be is blank.

Does anyone know how I can fix this?

Cheers.