Image editor features>>Image orientation

Image orientation: left–original; center–30° CCW rotation; right–flopped .

Image editors are capable of altering an image to be rotated in any direction and to any degree. Mirror images can be created and images can be horizontally flipped or vertically flopped. A small rotation of several degrees is often enough to level the horizon, correct verticality (of a building, for example), or both. Rotated images usually require cropping afterwards, in order to remove the resulting gaps at the image edges.

source : http://en.wikipedia.org/wiki/Digital_image_editing#Basics_of_image_editing