Yup. There's no "just the highest res possible" option.
Of course, there are reasons for this (the 5 MP is used not just for shares, but also for editing or viewing in apps to help conserve device resources), but it does prevent you from using the camera at the advertised "21 MP" resolution.
(Apparently it's 20 MP according to phonearena, but I distinctly remember when I bought it that the advertised res was 21 MP. However, looking through my camera roll on OneDrive, it appears every picture I've taken to date has been 5 MP. -_- Oh well, serves me right for never looking into it before.)