1. A method for processing a 2D or 3D reconstruction image, the method comprising: recording, by a magnetic resonance device, the 2D or 3D reconstruction image, the magnetic resonance device including a gradient coil to generate a gradient field; distortion-correcting a nonlinearity of the 2D or 3D reconstruction image to generate an image distortion of the gradient field using an algorithm that processes measurement signals at various measuring points lying in an imaging volume, the algorithm including a first input value for each of the measurement signals, the first input value describing a real gradient field given at a real measuring point of the signal; and back-transforming the distortion-corrected reconstruction image into a distortion-uncorrected reconstruction image by using the algorithm.