Encryption is a very important way to secure data in storage and communication, and it is a process of encoding messages or information in such a manner that only authorised persons can access it. Different techniques are used to protect confidential image data against illicit access. In image encryption using chaotic systems, most authors use or design algorithms to generate the initial parameters’ values from the secret key. However, as the key size depends on the number of these parameters, the used algorithms show little sensitivity to small changes in the key. To enhance both security and sensitivity in the choice of the initial parameters, this work combines the use of the Grain‐128a stream cipher algorithm with two‐dimensional Zaslavsky chaotic map. Firstly, the Grain‐128a algorithm is applied to generate the required parameters of Zaslavsky's chaotic map from a fixed length 256‐bit secret key. Secondly, the sequences generated by the chaotic map are used to encrypt the image using a bit confusion and diffusion process. The simulation results on greyscale, colour, binary, indexed, and medical images together with the scores obtained in the evaluation of the algorithm show that the proposed method is very sure and effective in encrypting images of any size and any type.