The numerical simulation of contrast is used for studying the double-crystal images of dislocations and precipitates in gallium arsenide crystal. It has been found that the images of dislocations in gallium arsenide crystals are dominated by a 'rosette' due to the dilatation-orientation contrast which corresponds to the deformation field. Some interference fringes, present in the plane-wave images, disappear in the simulations taking into account the beam divergence. It was confirmed that simulations taking into account beam divergence provide more realistic approximation of the experimental images. It was found a number of characteristic features differing the images of dislocations from those of small precipitates.