We evaluate performance characteristics of LSB steganalysis based on statistics of image difference signal using a discretized version of two dimensional Gaussian distribution. We numerically obtain a joint probability and a distribution of difference signal for our probabilistic model. Furthermore, a variation of difference signal by zero embedding and that by random embedding are computed. Many results of our computer simulation show a limit of estimation ability. From the point of view of the mean squared error, we argue that our method is superior to sample pairs analysis and RS steganalysis. The miss rate and false alarm rate for our steganalysis are plotted.