  We explore the observational appearance of the photosphere of an
ultrarelativistic flow with internal dissipation of energy as predicted by the
magnetic reconnection model. Previous study of the radiative transfer in the
photospheric region has shown that gradual dissipation of energy results in a
hot photosphere. There, inverse Compton scattering of the thermal radiation
advected with the flow leads to powerful photospheric emission with spectral
properties close to those of the observed prompt GRB emission. Here, we build
on that study by calculating the spectra for a large range of the
characteristics of the flow. An accurate fitting formula is given that provides
the photospheric spectral energy distribution in the ~10 keV to ~10 MeV energy
range (in the central engine frame) as a function of the basic physical
parameters of the flow. It facilitates the direct comparison of the model
predictions with observations, including the variability properties of the
lightcurves. We verify that the model naturally accounts for the observed
clustering in peak energies of the E*f(E) spectrum. In this model, the Amati
relation indicates a tendency for the most luminous bursts to have more energy
per baryon. If this tendency also holds for individual GRB pulses, the model
predicts the observed narrowing of the width of pulses with increasing photon
energy.
