A device has been reported recently in which electrons transit through the image states of a negative-electron-affinity glass substrate before being emitted to the vacuum. The external field required for this emission may be as low as 10 V/cm, which is up to three orders of magnitude lower than the fields encountered with other materials. In order to address what appears to be an essential aspect of this device, we present a modeling of field-assisted emission from the image states of a dielectric substrate. The analysis includes a characterization of the image states and considers direct tunneling and thermal excitation to higher energy levels as possible mechanisms for the emission. The model turns out to provide surface charge densities and emission currents that are in excellent agreement with experiments. For the working conditions of the device, the simulations show that the emission has a dominant thermally enhanced field emission component and that the image states play a significant role in pinning the Fermi level to values that are close to the vacuum level.