Frequency distributions are particularly difficult to analyze by means of image processing systems if the digital images to be processed are divided into disjoint image areas and signal processors associated with each image area perform pre-processing of the pixel data. To determine a total frequency distribution, a large number of corresponding table entries must be transferred to a central processor. The invention accordingly proposes that the individual signal processors perform summation of the individual entries in a frequency distribution to obtain a cumulative frequency table. To find a p-quantile, not all the entries of the cumulative frequency tables need to be transferred to the central processor. If the latter in particular performs interval nesting, a value from the cumulative frequency tables only needs to be interrogated a manageable number of times in order to obtain a single entry in the total image cumulative frequency table.