Questions about modding

[Apeiron]

Hi everybody. I'm back in civ 4 and I'm trying to code a modmod for FFH2 : Paradeigma.

My first question is : how to create a picture for a new opening menu ?
More precisely I succeeded to create my new opening menu :

<MainMenu>
<Type>MAIN_MENU_PARADEIGMA</Type>
<Description>TXT_KEY_MAIN_MENU_PARADEIGMA</Description>
<Scene>MAINMENU_SCENE_PARADEIGMA</Scene>
<SceneNoShader>MAINMENU_SCENE_PARADEIGMA</SceneNoShader>
<Soundtrack>AS2D_CANTAR_DE_PROCELLA</Soundtrack>
<Loading>MAINMENU_LOAD_BTS</Loading>
<LoadingSlideshow>MAINMENU_SLIDESHOW_LOAD_BTS</LoadingSlideshow>
</MainMenu>

And I succeeded for the text and the soundtrack.
For the picture I created that :

<InterfaceArtInfo>
<Type>MAINMENU_SCENE_PARADEIGMA</Type>
<Path>Art/Interface/Main Menu/Paradeigma/CIV4MainMenuBG.nif</Path>
</InterfaceArtInfo>

I have ever created a 512*256 picture (converted in .dds) and a folder "Paradeigma" in the "Main Menu" folder, but I don't understand what I have to do with it and what is really a .nif file.