Modding help?

[lukerduker123]

Alright, I'm working on a little niche mod for a buddy of mine, but I've ran into two, maybe three problems.

First problem, how do LBMs work? That is, how do I change them? I want to add a few new pictures to the game, but I don't know how.

Secondly, how do I add new unit sprites to the game? I tried to use the whole conversion tool but that turned into a cluster f-. Do I have to use a specific colour palette for sprites or something? And what do I do to import then? Oh, and how do turrets work? I'm using Paint.net for the BMP files-- is that what I should use?

Thirdly, how do I change the flag of a nation?

Thanks for the help!