How hard can it be to add a "rotate" action?
Edit: As of 2016 this functionality is now part of iOS's native photo app
The following information pertains an iPhone, but I assume the iPad is identical in terms of features.
Short Answer: You can’t rotate photos in the default iOS’s Camera Software; you need to install a third party utility, like PS Express (recommended and free):
You can do this in the iOS photos app by tapping on the edit button on the toolbar at the bottom when viewing a photo, then choosing crop/rotate, then rotate.