Random walk theory is used to calculate the line spread function (LSF) of photons as they cross the midplane of a slab of finite thickness. The relationship between the LSF and the photon transit time in transillumination time‐resolved experiments is investigated. It is found that the LSF is approximately Gaussian distributed, with a standard deviation, σ, which can be used as a criterion of the spatial resolution of the imaging system. Results are substantiated by comparison with actual data in the literature. Any given resolution can be improved by reducing the excess transit time Δt, but heterogeneity of the scattering medium and low levels of detected light enormously complicate the achievement of subcentimeter spatial resolution. The latter point is discussed by using optical parameters of breast tissues for visible and near‐infrared radiation (NIR) light.