Steganography is a system that hides information in an application cover carrier like image, text, audio, and video. Considerable amount of work has been carried out by different researchers on this subject. Least Significant Bit (LSB) insertion method was more suspicious and low robustness against attacks. The objectives of this study were to analyse various existing system and implement a dynamic substitution based Image Steganography (IS) with a secret key. Our proposed method is more difficult to attack because of message bits are not inserted in to the fixed position. In our method, the message bits are embedded into deeper layer depending on the environment of the host image and a secret key resulting increased robustness. The robustness specially would be increased against those intentional attacks which try to reveal the hidden message.