Public/GUI/GuiElements[Affinities].xml.

Small & medium are the population icons, Large is the image that appears when you hover over the population, Leader is the image that represents the faction in diplomacy, Loading is the loading screen, and Mood is for the little lore popup at the beginning of the game.

There are other mods you could use that would let you see what image size is needed.

I believe Cali's hero creator tool lets you also customize faction appearances but I genuinely can't remember.

Thanks but I still want to keep the existing faction imagery just the custom one. But at least I have a starting point thanks

You cannot change the image of a custom faction without making an entirely new affinity, which is probably more than what youre asking for.