Octadecyltriethoxysilane (OTE) monolayers on mica are imaged using force modulation microscopy (FMM) and dynamic force modulation (DFM). Nanoscopic areas of mica are produced within OTE layers and serve as an internal standard for contact stiffness measurements. The contact stiffness is systematically studied as a function of imaging medium and force modulation amplitude and frequency. Measurements taken in liquid media are found to reflect more accurately the viscoelastic properties of the sample, while imaging in air is perturbed by the capillary neck at contact. Increasing modulation amplitude increases the overall signal in FMM. However, extremely large amplitudes diminish the contrast difference between OTE and mica because the sensing depth is much higher than the monolayer. The measured contact stiffness is found to depend sensitively upon the modulation frequency because of the presence of several resonances within 10-50 kHz, which cause the image contrast to vary or to flip. Collective motion of the molecules under contact is most likely responsible for these resonances. The observed amplitude and frequency dependence also allows active control of FMM image contrast.