Despite being a complete neophyte to actual modding, I actually know the answer to this. Most Civ IV mods package their assets in massive .fpk files -- easier to package, easier to access, but not available to search via Windows folders. So the image you're looking for exists within one of the .fpk files. Not sure if you can download it from elsewhere (Caveman2Cosmos has a github with all of the files unpackaged, and I think DOC is the same, but I don't think the same is true of RFCE).