Requantization is a key technology for reducing the bit rate of a previously compressed data. When recompression ratio is high, the requantizer may cause unacceptable quality degradation. To improve the quality of the requantized image, an optimization scheme for the requantization codebook has been proposed. The proposed scheme constructs an optimal requantization codebook in an iterative manner for a given original quantization codebook of transmitter. The construction of codebook is iteratively repeated until they reach a local optimum solution. Our approach can be applied not only to the scalar quantization, but to any method which employs vector quantization-based system. Simulation results show that the optimized system based on the proposed algorithm outperforms the conventional system which is made without consideration of requantization. The proposed algorithm enables a reliable image communication over heterogeneous networks.