1. A method for processing a reconstruction image, the method comprising: recording the reconstruction image by a magnetic resonance device having a gradient coil to generate a gradient field; distortion-correcting the reconstruction image by an image processing device, the distortion correcting being with regard to a nonlinearity, the distortion correcting resulting in an image distortion of the gradient field, the distortion correcting uses a first algorithm configured to process a measurement signal at various measuring points of a plurality of measuring points lying in an imaging volume, the first algorithm including a first input value describing a real gradient field given at a real measuring point of the plurality of real measuring points of the measurement signal; back-transforming the distortion-corrected reconstruction image, by the image processing device, into a distortion-uncorrected reconstruction image, the back-transforming uses a second algorithm corresponding to the first algorithm, the second algorithm including a second input value associated with the measurement signal, the second input value being a fictitious gradient field value associated with a distorted measuring point at which the measurement signal is processed, and the second input value is one of raised and lowered by a nonlinear field component of the real gradient field compared with a linear ideal gradient field.