Stable nonlinear beams, both solitary waves (nematicons) and optical vortices, can form in a nematic liquid crystal due to a balance between the nonlinear, nonlocal response of the nematic and the diffractive spreading of the light beam. The `huge' nonlinearity of a nematic liquid crystal makes it ideal for the experimental development of photonic devices as nonlinear effects occur over millimetre distances. In this work, a simple and fast method to analyse the trajectory of a nonlinear beam within a finite liquid crystal cell, based on a classical method not explored in this context, the method of images, is developed. With the orientation of the nematic molecules modelled using images, the evolution of the beam is obtained by using both asymptotics and modulation theory. The efficiency of this new method is shown by comparisons with a standard Fourier series solution for the nematic response and full numerical solutions of the governing equations. It is found that only a small number of images is required compared with the usual Fourier series technique in order to obtain excellent agreement with full numerical solutions. Finally, the contrasting effect of the cell boundaries on a nematicon and a vortex is explored.