1. A method for processing a 2D or 3D reconstruction image, recorded by a magnetic resonance device including a gradient coil to generates a gradient field, and distortion-corrected with regard to a given nonlinearity, leading to an image distortion—of the gradient field by using an algorithm that processes the measurement signals at various measuring points lying in an imaging volume, the algorithm processing, with respect to each signal processed by the algorithm, a first input value describing a real gradient field given at a real measuring point of the signal, the method comprising: back-transforming the distortion-corrected reconstruction image into a distortion-uncorrected reconstruction image by using the first algorithm or a second algorithm corresponding thereto and to which there is given, as a second input value in relation to each signal processed by it, one such as describes a fictitious gradient field at the respective distorted measuring point at which the processed signal appears, and is raised or lowered by the nonlinear field component of the real gradient field compared with the linear ideal gradient field.