This article tackles the problem of reconstructing the 3D shape model of asteroids and cometary nuclei from images obtained with a visible imaging system aboard a planetary spacecraft. We describe a photoclinometry method based on the optimization of the chi-square difference between observed and synthetic images of the object by deformations of its initial shape, described here as a mesh of triangular facets. The non-linear optimization is performed using the so-called “limited-memory Broyden-Fletcher-Golbfarb-Shanno” algorithm. The deformations can be applied: (i) by modifying the coefficients of a spherical harmonics expansion in order to extract the global shape of the object, and/or (ii) by moving the height of the vertices of a triangular mesh in order to increase the accuracy of the global shape model and/or to derive local topographic maps of the surface. This method has been tested on images of the asteroids Steins and Lutetia obtained by the imaging system aboard the Rosetta spacecraft of the European Space Agency.