To reduce the dependency of image encryption algorithm on the initial keys and to improve its abilities in resistance to exhaustive attack, statistical attack and differential attack, a modified image encryption algorithm used for encrypting the rectangular grayscale of object was presented, which was obtained by permutation and diffusion synchronously. Firstly, rearranged the image pixels through scanning. A chaotic sequence generated from double logistic map was operated on the plaintext image. Finally, diffuse the scrambled image by a method based on double-direction combination between plaintext and ciphertext that was controlled by Chebyshev-Logistic chaotic sequence. Analysis including key space, statistical histogram, correlation of adjacent pixels, information entropy test, and diffusion test was made to evaluate the performance of this algorithm, simulation results show that such algorithm is very dependent on the initial-values, it extends the key space effectively, and improves the image's ability of encryption algorithm in resistance to exhaustive attack, statistical attack and differential attack effectively.