Tip-enhanced Raman spectroscopy (TERS) is a powerful method for imaging vibrational motion and chemically characterizing surface-bound systems. Theoretical simulations of TERS images often consider systems in isolation, ignoring any substrate support, such as metallic surfaces. Here, we show that this omission leads to deviations from experimentally measured data, as demonstrated by simulations with a new finite-field periodic formulation of first-principles simulation of TERS spectra that can address extended systems. We show that TERS images of defective MoS2 monolayers calculated using cluster models are qualitatively different from those calculated when accounting for the periodicity of the substrate. For Mg(II)-porphine on Ag(100), a system for which a direct experimental comparison is possible, these simulations prove to be crucial for explaining the spatial variation of TERS intensity patterns and allow us to uncover fundamental principles of TERS spectroscopy. We explain how and why surface interactions affect images of out-of-plane vibrational modes much more strongly than those of in-plane modes, thereby providing an important tool for the future interpretation of these images in more complex systems.