1. An image processing apparatus comprising: skin-tone image area detecting means for detecting a skin-tone image area, which is contained in an image represented by applied image data, based upon image shape or image structure; gray pixel detecting means for detecting gray pixels contained in the image represented by the image data based upon color information obtained from pixels that constitute the skin-tone image area that has been detected by said skin-tone image area detecting means; and color temperature estimating means for estimating color temperature of a photographic light source based upon gray pixels that have been detected by said gray pixel detecting means; wherein said gray pixel detecting means includes: first coefficient multiplying means for multiplying a representative value of color information obtained from pixels constituting the skin-tone image area by prescribed coefficients; first chromaticity value converting means for converting the coefficient-multiplied skin-tone-area representative value, which has been calculated by said first coefficient multiplying means, to a representative chromaticity value; determination means for determining whether the coefficient-multiplied skin-tone-area representative chromaticity value obtained by said first chromaticity value converting means lies within a zone in the vicinity of a skin-tone—blackbody locus; provisional color temperature estimating means for estimating that a color temperature corresponding to a point on the skin-tone—blackbody locus nearest the coefficient-multiplied skin-tone-area representative chromaticity value is a provisional color temperature if said determination means has determined that the coefficient-multiplied skin-tone-area representative chromaticity value lies within the zone in the vicinity of the skin-tone—blackbody locus; second coefficient multiplying means for multiplying color information, which is obtained from each pixel constituting the image represented by the image data, by the prescribed coefficients that will cause the coefficient-multiplied skin-tone-area representative chromaticity value to fall within the zone in the vicinity of the skin-tone—blackbody locus; and second chromaticity value converting means for converting a plurality of coefficient-multiplied pixel values, which have been obtained by said second coefficient multiplying means, to chromaticity values; wherein from among the pixels that constitute the image represented by the image data, there are specified pixels for which the coefficient-multiplied pixel chromaticity values obtained by said second chromaticity value converting means will fall within a zone in the vicinity of a point on a gray—blackbody locus corresponding to the provisional color temperature that has been estimated by said provisional color temperature estimating means, and the specific pixels are detected as gray pixels.