Showing too much 'skin' detection in software

Your best bet is to deal with the image in the HSV colour space (see here for rgb - hsv conversion). The colour of skin is pretty much the same between all races, its just the saturation that changes. By dealing with the image in HSV you can simply search for the colour of skin.

You might do this by simply counting the number of pixel within a colour range, or you could perform region growing around pixel to calculate the size of the areas the colour.

Edit: for dealing with grainy images, you might want to perform a median filter on the image first, and then reduce the number of colours to segment the image first, you will have to play around with the settings on a large set of pre-classifed (adult or not) images and see how the values behave to get a satisfactory level of detection.

EDIT: Heres some code that should do a simple count (not tested it, its a quick mashup of some code from here and rgb to hsl here)

Bitmap b = new Bitmap(_image);BitmapData bData = b.LockBits(new Rectangle(0, 0, _image.Width, _image.Height), ImageLockMode.ReadWrite, b.PixelFormat);byte bitsPerPixel = GetBitsPerPixel(bData.PixelFormat);byte* scan0 = (byte*)bData.Scan0.ToPointer();int count;for (int i = 0; i < bData.Height; ++i){    for (int j = 0; j < bData.Width; ++j)    {        byte* data = scan0 + i * bData.Stride + j * bitsPerPixel / 8;        byte r = data[2];        byte g = data[1];        byte b = data[0];        byte max = (byte)Math.Max(r, Math.Max(g, b));        byte min = (byte)Math.Min(r, Math.Min(g, b));        int h;        if(max == min)            h = 0;        else if(r > g && r > b)            h = (60 * ((g - b) / (max - min))) % 360;        else if (g > r && g > b)            h = 60 * ((b - r)/max - min) + 120;        else if (b > r && b > g)            h = 60 * ((r - g) / max - min) + 240;        if(h > _lowerThresh && h < _upperThresh)            count++;    }}b.UnlockBits(bData);

Of course, this will fail for the first user who posts a close-up of someone's face (or hand, or foot, or whatnot). Ultimately, all these forms of automated censorship will fail until there's a real paradigm-shift in the way computers do object recognition.

I'm not saying that you shouldn't attempt it nontheless; but I want to point to these problems. Do not expect a perfect (or even good) solution. It doesn't exist.

I doubt that there exists any off-the-shelf software that can determine if the user uploads a naughty picture. Your best bet is to let users flag images as 'Adults Only' with a button next to the picture. (Clarification: I mean users other than the one who uploaded the picture--similar to how posts can be marked offensive here on StackOverflow.)

Also, consider this review of an attempt to do the same thing in a dedicated product: http://www.dansdata.com/pornsweeper.htm.

Link stolen from today's StackOverflow podcast, of course :).