Through an interplay between classical dynamic at room temperature (RT) and static STM simulations of a given adsorbate, we set up the calculation of an STM contrast acquired at finite temperature. We apply this new procedure on the model case of a molecular poison corrupting Pt catalysts. Despite significant oscillations in the time evolution of the poison coordinates, the dynamic structure is found identical to the equilibrium geometry. Similarly, the RT STM image determined over averaging along the trajectory is undistinguishable from the frozen image. In this context, allowing for thermal excitation of a chemisorbed moiety does not appreciably change its STM contrast. (C) 2003 Elsevier B.V. All rights reserved.