1. An image processing method for expressing small regions in an image with not more than a predetermined number of representative colors, in which an image processing is carried out for each of the small regions, the method comprising: a first selection step of, with respect to a plurality of pixels in the small region, based on variation values that indicate the extent of variation, in the small region, of values of a plurality of color variables that express a color of each of the pixels, selecting one of the color variables as a target color variable; a first reference value calculation step of, from among values from a maximum value to a minimum value of the target color variable in the small region, when there is a value of the target color variable for which the number of pixels is “0”, obtaining the value of the target color variable as a first reference value for grouping the pixels; a first grouping step of grouping the pixels in the small region based on the first reference value obtained in the first reference value calculation step; and a first representative color calculation step of, for each of groups in the first-grouped small region, obtaining a representative color based on a value of each of the color variables of pixels that belong to the group, wherein said plurality of color variables includes a luminosity that has been corrected to increase the respective value of the luminosity when the originally obtained value for a specific pixel exceeds a set threshold.