Steganography is one of the branches of information security field, it aims to hide information inunremarkable cover media so as not to arouse an eavesdropper’s suspicion. The secret message is hiddenin such a way that no significant degradation can be detected in the quality of the original image. The aimof this paper is to introduce an efficient steganographic scheme to hide data over gray scale images. Thisscheme is based on the property of the human eye, which is more sensitive to the change in the smooth areathan the edge area using pixel value difference, besides employing the LSB substitution technique as afundamental stage. The experimental results show that the proposed method could successfully achieve thegoals of the high embedding capacity and maintaining the visual quality, in addition, provides more securedata hiding using selective pixel positions determined by a secret image (i.e. key). Moreover, based on that,the secret message is replaced with dynamic LSBs, our scheme can effectively resist several imagesteganalysis techniques.