Secret image sharing is a useful way to protect a secret image by sharing it to several shadow images. The proposed scheme is an improvement of the secret image sharing method, in order to prevent the shadow images from revealing the shape of the original secret image. For the reason that the adjacent pixels of an image is highly close, the shadow images, the result of the general image sharing process, always give out the shape information of the secret which can be the main target of rivals. An improved method to solve this problem is using a function, also called the key, to permute the pixels of the original image before sharing process, but the custody of the key brings new troubles. The proposed scheme offers a special method to generate the key using the secret image itself, so there is no need to safeguard the key.