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Similarly, if you have a clean reference of the source image, you could observe that the first N pixels are modified and the rest are not, and conclude that something is hidden in them.
If the pixels are selected at random, this analysis is less conclusive. Ideally you'd hide a random value in every pixel so that all pixels are manipulated equally.
You should also give thought to how robust you want the stegosystem to be to image filtering, downsampling, etc. The MPAA uses watermarks on movies to determine which theaters are the source of camrips, and those watermarks have to be as robust as possible (they must survive both the camera's inherent reduction in quality and possible transcoding/editing by the pirates). Not all stegosystems need to be robust, for example SDMI used one robust and one weak in an effort to identify downsampled copies of the audio (the RIAA assumed that downloaded music would always be significantly downsampled).