A method for carrying out a step of extracting a portion of a predetermined size from the image data, and a step of preparing a predetermined number of compressed partial data compressed with different compression parameter values by repeating preparation of compressed partial data by compressing the partial data in predetermined format for the predetermined number of times with the compression parameter value used in the predetermined format changed for each compression, on the entire image data. The method also calculates the total of the data sizes of all of the compressed partial data compressed with the same compression parameter value for each of the compression parameter values and prepares the compressed image data file from compressed partial data compressed with a compression parameter value for which the total is not larger than a given threshold and which is the closest value to the threshold.