We describe some basic optical image-processing operations with acousto-optic (AO) Bragg diffraction. Instead of using frequency-plane filters, we place an AO cell behind the object. We then realize experimentally one-dimensional edge enhancement, which utilizes a high-pass filtering effect in the undiffracted order from the AO cell. A numerical simulation compares well with the experimental results. With two AO cells oriented orthogonally to each other, a second-order mixed derivative operation, evident from the four-corner enhancement of a square, is also demonstrated.