Image editor features>>Perspective correction and distortion

Some image editors allow the user to distort (or "transform") the shape of an image. While this might also be useful for special effects, it is the preferred method of correcting the typical perspective distortion which results from photographs being taken at an oblique angle to a rectilinear subject. Care is needed while performing this task, as the image is reprocessed using interpolation of adjacent pixels, which may reduce overall image definition. The effect mimics the use of a perspective correction lens, which achieves a similar correction in-camera without loss of definition.

Perspective correction: left–original, uncorrected
right–perspective distortion removed.

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