1. An image processing method, comprising: obtaining image data, the image data being data showing an image and being formed of a plurality of pixel data, the image being formed of a plurality of pixels arranged two-dimensionally, the pixel data being data showing a tone of a pixel; determining a plurality of defective pixels from among an image indicated by the image data, determining a pixel of interest from the plurality of defective pixels, determining a plurality of peripheral pixels that surround the pixel of interest and that are located within a region of a predetermined size; changing pixel data of peripheral pixels by performing a median filtering process, on each peripheral pixel, the median filter process being a process that: obtains a median tone value from respective pixel data of a plurality of pixels, except for the defective pixel(s), that surround a pixel to be processed and that are located within a region of the predetermined size, and changes the pixel data of the pixel to be processed to the obtained median tone value; and changing pixel data of the pixel of interest, by performing the median filter process on the pixel of interest on the basis of the peripheral pixels that each have been subjected to the median filter process, wherein when determining the defective pixels, an isolation point is extracted from the original image, a rectangular region that surrounds the isolation point is determined, when a ratio of an area occupied by the isolation point to an area of a square whose side length is equal to a longer side length of the rectangular region is larger than a predetermined value, when an area of the isolation point is smaller than a first threshold, a defective pixel is discriminated as present in a position of the isolation point in the original image, and when the ratio is smaller than the predetermined value, and when the area of the isolation point is smaller than a second threshold that is larger than the first threshold, the defective pixel is discriminated as present in the position of the isolation point in the original image.