We show in this paper that the contrast of the interface between resonant and nonresonant media imaged in Coherent anti-Stokes Raman scattering (CARS) microscopy strongly depends on the pump and Stokes fields spectral detuning. More specifically, when this detuning drives the vibrational resonance with the maximum phase difference, a spatial dip appears at the interface in the CARS image. This effect is studied both theoretically and experimentally and is an evidence of the coherent and resonant nature of the CARS contrast mechanism.