Steganography is an ancient art of conveying messages in a secret way that only the receiver knows the existence of message. So, a fundamental requirement for a steganographic method is imperceptibility# this means that the embedded messages should not be discernible to the human eye. There are two other requirements, one is to maximize the embedding capacity, and the other is security. The least-significant bit (LSB) insertion method is the most common and easiest method for embedding messages in an image. However, how to decide on the maximal embedding capacity for each pixel is still an open issue. An image steganographic model is proposed that is based on variable-sized LSB insertion to maximise the embedding capacity while maintaining the image fidelity. For each pixel of a gray-scale image, at least 4 bits can be used for messages embedding. Three components are provided to achieve the goal. First, according to contrast and luminance characteristics, the capacityeval..