There are numerous halo mapfile editing tools out there.
Several of them allow you to extract and reinsert bitmaps/textures.
Extract the menu bitmaps from the ui.map:
menu_load_campaign.dds
menu_multiplayer.dds
menu_settings.dds
menu_game_demos.dds
Or the halo logo:
halo_logo.dds
Edit using Photoshop.
Be sure to save using same dds format and keep
alpha channel masks if they originally had them.
Use the map tool to reinsert the modded menus.
I don't know if anyone has managed to add another menu item that is functional
on the xbox. You can replace the game demos, and swap the xdemos.xbe for
whatever you want (I launch HCE to edit the cache), but I don't know
about a fifth menu item for the xbox halo menu, yet.