1. An image processing method of converting a first image into a second image, comprising: providing multiple first dictionaries produced by dictionary learning and multiple second dictionaries corresponding to the first dictionaries; performing, on each of the multiple first dictionaries, a process to approximate the first image by linear combination of elements of the first dictionary so as to produce a linear combination coefficient and thereby acquiring multiple linear combination coefficients; calculating, for each of the multiple linear combination coefficients, a ratio between a largest coefficient element whose absolute value is largest among coefficient elements of the linear combination coefficient and a second-largest coefficient element whose absolute value is second-largest thereamong and selecting a specific linear combination coefficient in which the ratio is largest among the multiple linear combination coefficients; selecting, from the multiple second dictionaries, a specific dictionary corresponding to the first dictionary for which the specific linear combination coefficient is produced; and producing the second image by using linear combination of the specific linear combination coefficient and elements of the specific dictionary.