1. A method of reducing block noise, mosquito noise, and other image noises in an image by a filtering process using a smoothing filter, said block noise, mosquito noise, and other image noises being caused at the time of decoding encoded, compressed image data on a block-by-block basis, the method comprising: changing the intensity of the image noise reduction in said filtering process in a continuous or stepwise manner according to an output size of an image to be outputted to printer paper, photographic paper or other output media, thereby allowing the intensity of the image noise reduction to be increased as the output size of the image increases; dividing each of luminance component image data and color-difference component image data of said image data into blocks corresponding to the blocks created in the encoding and decoding of the image data; applying a first filtering step to the luminance component image data with each of pixels on the boundaries of the blocks being designated as a target pixel, thereby creating a first luminance component image data with the boundaries of the blocks smoothened; performing a second filtering step with each pixel of the first luminance component image data being designated as a target value, thereby creating a second luminance component image data with its entirety smoothened; creating edge image data by subtracting each pixel value of the second luminance component image data from a corresponding pixel value of the first luminance component image data; creating corrected edge image data with each difference value of the edge image data corrected under given conditions; creating a third luminance component image data by adding each offset value of the corrected edge image data to its corresponding pixel value of the second luminance component image data; applying a third filtering step to the color-difference component image data with each pixel of the color-difference component image data being designated as a target pixel, thereby creating a first color-difference component image data with its entirety smoothened; and changing the filtering coefficient of the smoothing filter used in said second filtering step, thereby changing the intensity of image noise reduction in said second filtering step, wherein the above steps are performed by a processor, and the filtering process is applied to image data stored in computer memory.