This paper proposes a novel image encryption system based on a new simultaneous permutation and diffusion framework utilizing modified Josephus traversing (SPDMJT). Firstly, SPDMJT utilizes the reverse scanning method to perform encryption process, guaranteeing the ergodicity of scrambling. Secondly, the diffusion is embedded into the Josephus traversing, preventing attackers from targeting the scrambling and diffusion phases separately. Thirdly, in order to enhance permutation effect, modified Josephus traversing employs a dynamic scrambling method where the scrambling scheme of the current pixel depends on the value of the previous diffused pixel. Moreover, SPDMJT adopts plaintext correlation both in scrambling and diffusion operation, which strikes a balance between ciphertext sensitivity and plaintext sensitivity, offering resistance against chosen-plaintext attacks (CPA), noise attacks and data loss. Simulation results show that the proposed algorithm has high encryption efficiency and can withstand various common attacks.