Recently,two chaotic image encryption schemes have been proposed,in which shuffling the positions and changing the grey values of image pixels are combined.This paper provides the chosen plaintext attack to recover the corresponding plaintext of a given ciphertext.Furthermore,it points out that the two schemes are not sufficiently sensitive to small changes of the plaintext.Based on the given analysis,it proposes an improved algorithm which includes two rounds of substitution and one round of permutation to strengthen the overall performance.