The banners and titled/info for Wii games are actually taken from the save files instead of the games themselves, which means you will need to make the save file ID different from the standard Mario Kart Wii if they aren't already and then replace the image used in the save file. This requires a few modifications to the ISO. I'm actually trying to do this myself, but haven't been able to do it successfully yet. If anyone has more knowledge on how to do this that would be great, as I know it's possible.
If not I'll just have to play around with Wiimms ISO Tools until I figure it out.
If not I'll just have to play around with Wiimms ISO Tools until I figure it out.