1. A method of scanning image files for pornographic image content, the method comprising: maintaining a list of items of copyright information deemed to indicate that the image file is one of acceptable or unacceptable; and processing an image file by: examining any copyright data fields of the image file containing copyright information to detect the presence of any item of copyright information in said list; in the event that there is not detected the presence of an item of copyright information in said list, analysing the image content of the image file to detect the presence of pornographic image content, and outputting data indicating that the image file does or does not contain pornographic image content on the basis of the analysis; in the event that there is detected the presence of an item of copyright information deemed to indicate that the image file is acceptable, outputting data indicating that the image file does not contain pornographic image content, without analysing the image content of the image file; and in the event that there is detected the presence of an item of copyright information deemed to indicate that the image file is unacceptable, outputting data indicating that the image file does contain pornographic image content, without analysing the image content of the image file.