A key rotation multiplexing method to the modified joint transform correlator arrangement is introduced. For encryption, the rounding key with a certain rotation angle is assigned to each image to be encrypted to generate a corresponding joint power spectrum. Then the final encryption result, namely multiplexed joint power spectrum, will be obtained by summing the above mentioned joint power spectrum. The decryption of the multiplexed joint power spectrum with the correct rotation angle of the additional key will show the corresponding primary images. Theoretical analysis and computer simulations reveal that rotation angle of the additional key is a valid parameter to perform image multiplexing encoding with the modified joint transform correlator architecture. Also, computer simulations to investigate how the number of encrypted images affects the decryption quality are performed.