The process of high-gain image amplification in photorefractive crystals is investigated. We show that in the regime of large energy transfer, the spatially nonuniform depletion of the pump wave results in the introduction of distortions into the amplified output image. This distortion mechanism and its dependence on the characteristics of the image to be amplified are described theoretically through numerical solutions of the coupled-wave equations for image-bearing beams. The numerical results are then checked experimentally in photorefractive BaTiO_3.