1. A computer based method for evaluating a graphic image file for the presence of objectionable visual content, the graphic image file comprising a number of pixels each having spectral components defined in accordance with a color model, the method comprising steps of: (a) evaluating relative weighting of the spectral components of each pixel in turn to classify each said pixel as either not representing human skin or as possibly representing human skin; and (b) processing the graphic image file in relation to the resulting classification of each said pixel during the evaluating step (a); wherein the color model comprises a numeric scheme so that the spectral components of each pixel are expressed as the equivalent of a set of red, green and blue (R, G, B) components corresponding to the respective amounts of red, green and blue in each pixel; and further wherein each of the (R, G, B equivalent) components has a numeric value ranging from m to n, and wherein the evaluating step (a) comprises a step of comparing the respective numeric values of the (R, G, B equivalent) components; and further wherein said pixel is classified as not representing human skin when at least one of the following conditions is satisfied: the numeric value of the R,G,B equivalent blue component is greater than the numeric value of the R,G,B equivalent red component; the numeric value of the R,G,B equivalent blue component is greater than the numeric value of the R,G,B equivalent green component; the numeric value of the R,G,B equivalent green component is greater than the numeric value of the R,G,B equivalent red component; the numeric value of the R,G,B equivalent blue component is less than one-quarter the numeric value of the R,G,B equivalent red component; and the numeric value of the R,G,B equivalent blue component exceeds a threshold T.