Try using iFile or iFunBox? I've never used iFunBox, but from what I hear, it works well for normal iOS users. I would help you search, but with the release of Animal Crossing: New Leaf being tomorrow, I don't have the time to search the file system. I'd assume it would be found using this route:
*Era* > Documents > Levels > Images > Era > *File name*
I'd assume the file name would be something like "Era_icon-sparhat2" or "Era-icon_sparhat2". Try searching up those, the filesystem isn't necessarily the most organized thing, but then again, it wasn't meant to be used by normal players, I guess.