Images can be encrypted in spatial domain, frequency domain, or both. Most of the image encryption algorithms operate in spatial domain. Encryption in frequency domain has an advantage of being resistante to many image processing operations. This paper presents an image encryption algorithm in frequency domain using a chaotic permutation. Because of the images represented in JPEG format, then to increase robustness to the image processing operations, the plain images are encrypted in blocks 8 × 8. After an image is encrypted, the cipher-image was modified by some common image processings. The experiments shows that the cipher-image was robust to the image processing operation, the decrypted images can be still recognized well.