If the cover image is a solid block of colour, you can detect the presence of the steganography by noting that the first N pixels vary slightly, and the rest ar

If the cover image is a solid block of colour, you can detect the presence of the steganography by noting that the first N pixels vary slightly, and the rest are solid. 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.