See Sovereign's post.

The rank images system has three stages:

1. Memberlevel (Junior/Full/Senior)
2. Primary usergroup override(Admin/Moderator/Senior Moderator/Banned/Temp Banned/Loser/Guest/Whipping Boy/News Poster/Advisor)
3. Userid override (Webmaster/Owner)

By using the userid override, I can make anyone have any rank image I like. My access-test account never has AdminCP access, yet has the same Webmaster image as I do, because there is no Webmaster usergroup; this badge is assigned by ID.