Image steganography is a covert communication method that uses an image as the cover to hide the truth from potential attackers that some secret message hidden in the image is being transported. When we appreciate the astonishing beauty of a world famous picture in its digital form on the computer, it is hard to imagine that the picture might actually be working as a messenger, carrying some invisible important secret message along with it. In other words, steganography is a collection of cryptographic techniques that provide protection to the secret message by offering it the appearance of an image. In this survey paper, our focus is on the development and current status of steganographic techniques for grayscale images. We shall separately introduce schemes with high hiding capacities and schemes with high imperceptibility. The advantages and disadvantages of those schemes will be closely analyzed, offering directions for our future research efforts.