1. A method for improving the quality of an image including a plurality of pixels by enhancing the contrast of the image, comprising the steps of: outputting from an electrical imaging device a first electrical signal having image information therein, the image information including pixel and gray-scale level information; receiving the electrical signal from the electrical imaging device; selecting a first pixel and a first gray-scale level at the first pixel from the image information; selecting a second pixel and a second gray-scale level at the second pixel from the image information; subtracting the gray-scale level of the image at the first pixel from the gray-scale level of the image at the second pixel, so as to produce a difference .DELTA.G; multiplying the differences .DELTA.G by N, so as to produce a value .DELTA.GN which is N times as large as the difference .DELTA.G, N being larger than 1; determining a gray-scale level of an image with enhanced contrast at the second pixel depending on the gray-scale level of the image with enhanced contrast at the first pixel and the value .DELTA.GN, wherein the determining step comprises the steps of: determining a sum of the gray-scale level of the image with enhanced contrast at the first pixel and the value .DELTA.GN as the gray-scale level of the image with enhanced contrast at the second pixel, when the sum is in the range from a first predetermined value K1 to a second predetermined value K2, the first predetermined value K1 being larger than the second predetermined value K2; determining the first predetermined value K1 as the gray-scale level of the image with enhanced contrast at the second pixel, when the sum is larger than the first predetermined value K1; and determining the second predetermined value K2 as the gray-scale level of the image with enhanced contrast at the second pixel, when the sum is smaller than the second predetermined value K2; and displaying a second signal having the enhanced contrast image information therein.