The paper presents the first experimental investigation of the nonlinear optical properties of graphene flakes, focusing on their strong third-order optical nonlinearity at near-infrared frequencies. The study uses four-wave mixing (FWM) to measure the nonlinear susceptibility, which is found to be approximately $10^{-7}$ esu and independent of the incident light wavelength. This property allows for high-contrast imaging of graphene flakes, with image contrasts orders of magnitude higher than those obtained using linear microscopy. The nonlinear response is compared to that of carbon nanotubes, showing a similar magnitude but with a dispersionless nature in graphene. The frequency independence of the nonlinear response is attributed to the dispersionless band structure of graphene. The research highlights the potential of using nonlinear optics for visualizing graphene flakes and suggests further exploration of these properties across a broader frequency range, including the THz range.The paper presents the first experimental investigation of the nonlinear optical properties of graphene flakes, focusing on their strong third-order optical nonlinearity at near-infrared frequencies. The study uses four-wave mixing (FWM) to measure the nonlinear susceptibility, which is found to be approximately $10^{-7}$ esu and independent of the incident light wavelength. This property allows for high-contrast imaging of graphene flakes, with image contrasts orders of magnitude higher than those obtained using linear microscopy. The nonlinear response is compared to that of carbon nanotubes, showing a similar magnitude but with a dispersionless nature in graphene. The frequency independence of the nonlinear response is attributed to the dispersionless band structure of graphene. The research highlights the potential of using nonlinear optics for visualizing graphene flakes and suggests further exploration of these properties across a broader frequency range, including the THz range.