Two things: You are not really adding noise, but just setting a pixel to black here and there. The idea of noise is to modify all color components of all pixels by adding a random value (plus or minus). So, start off by using a random number generator like rand(), scale its return value to the amount of noise you want and add the outcome pixel by pixel. You may think of it as computing a noise image and adding that image to your original image. If you want to refine that, don't use equally distributed random numbers, but gaussian distribution or others. And you may apply filterind (low-pass, higo-pass, etc) to the noise image before adding to modify the spectral distribution of noise.
