It is well established that lattice quantizers are highly suitable for encoding wavelet coefficients of still images. Existing lattices give rise to the variety of lattice quantizers available for quantization. In this paper we address the problem of selecting a lattice quantizer for a wavelet- based image coding system and show that the rate-distortion performance of a lattice quantizer can be well estimated based on the generalized Gaussian model of wavelet coefficients of images. Simplification of the rate-distortion function used for the estimation is achieved by utilization of the theta series with the 0.7-norm, which are derived in this paper. As shown by experimental results, our estimation closely matches the real rate-distortion performance of lattice quantizers.