This paper presents an application of AES (Advanced Encryption Standard) operations in image encryption and decryption. The encrypted cipher images always display the uniformly distributed RGB pixels. It looks like the noise of TV without signals. However, in some cases the ECB mode operation may appear a kind of pattern in the area related to the portion of the original image having the same color. The reason for the pattern generation and the four operation modes of CBC, CFB, OFB and CTR which have the features to eliminate the patterns are discussed, in terms of the degree of random noise in cipher image as well as the feasibility of parallel operation in cipher blocks for high speed processing.