The paper analyzes a symmetric image encryption scheme based on a new chaotic two-dimensional map. The map can encrypt image. The numbers of the left-map and the right-map are used as the secret keys of encryption. A diffusion mechanism was used to diffuse the pixels of image. But it isn’t enough safe. Firstly there are a lot of week keys and duplicate keys. Secondly the diffusion mechanism is too simple to ensure the information security. The paper designs a new method of keys generation. The keys are non-zero and random. It has a large keys space and avoids the duplicate keys. Then it takes parts of keys as the parameters of a classic chaotic map. The process is the new diffusion mechanism. It solves the problem of weak keys. Simulation results show the effectiveness of the method.