A novel image encryption scheme based on the modified skew tent map was proposed in this paper. In the key generating procedure, the algorithm generates a plaintext-dependent secret keys set. In the encryption process, the diffusion operation with cipher output feedback is introduced. Thus, cipher-image is sensitive to both initial keys and plaintext through only one round diffusion operation. The key space is large. As a result, the algorithm can effectively resist differential attacks, statistical attacks, brute-force attacks, known plaintext and chosen plaintext attacks. Performance test and security analysis demonstrates that this algorithm is eficient and reliable, with high potential to be adopted for secure communications.