does anybody know how you change the main menu image for a MOD?

MAINMENU_LOAD in Civ4ArtDefines_Interface.xml. Can be a static DDS or a Nif scene.
Note, that there are 3 or 4 similar tags, for the different main menu options you can choose - Warlords, BTS, Classic. Need to redirect them all, if you want to make sure, the user is seeing your menu, no matter what he uses. Don't remember them all, should be something like MAINMENU_LOAD_BTS, etc.

thanks for the tip!