We measured the X-ray fluorescence holograms of Si_0.999Ge_0.001, Si_0.8Ge_0.2, Si_0.5Ge_0.5, Si_0.2Ge_0.8 and Ge single crystals, and reconstructed the images of second neighbor atoms around Ge. The positional shift of the atomic image across the whole composition range was three times larger than the value predicted from the difference in the lattice constants of pure Si and Ge. We found that imaginary part of the reconstruction strongly affects the positions of the atomic images. Thus, using the negative real parts, the atomic image became sharp and its shift dependent upon Ge composition comes to the reasonable values.