Don't know if there's a post for this already, but as it's been mentioned multiple times by multiple people and isn't on the Known Issues list, I thought I'd bring it up.
The procedure can be replicated with any Sovereign or Champion.
1) Open up their profile page in the Hommonomonom and select "Edit Unit Card"
2) Edit the unit card (of course)
3) ??? (wait a few turns, or exit and reload the game)
4) PROFIT! (The unit card picture, after 5~ turns or so, or reloading, resets itself back to what it was in the first place. Sometimes it retains the new pose and background, but just the equipment changes back to the default. It is very inconsistent.)
It kind of makes this new feature useless, but it'd be really grand if it functioned as intended, for a number of reasons.