This paper presents a secure image transmission based on a chaotic synchronization. Chaotic systems present unpredictable and complex behavior. These characteristics, together with the dependence on the initial conditions as well as the tolerance of this type of systems, make theme ideal for transmission. In this paper a system of encryption based on chaotic signal is proposed. This system is used for the purpose of secure image transmission. For this purpose an algorithm for the encryption and the decryption process is proposed. In the proposed system, the transmitter mixes an input image with a chaotic signal at transmitter level using masking technique and a mixture function. The receiver must be able to reproduce exactly the same chaotic signal in order to subtract it from the received signal. This becomes possible with synchronization between the transmitter and the receiver: through the use of specific techniques, the trajectory of the Slave chaotic system can be bound to that of the Master system producing (almost) identical behavior. Simulation results demonstrate satisfactory performance, and robust against common attacks. The whole system is simulated in Matlab. The system works with both greyscale and color images.