  The radiation released at the transparency radius of an ultrarelativistic
flow can account for the observed properties of gamma-ray bursts (GRBs)
provided that sufficient energy is dissipated in the sub-photospheric region.
Here, I investigate how the peak energy of the E*f(E) spectrum and its overall
shape depend on the properties of the jet for various "dissipative
photospheres". I find that continuous energy release which results in electron
heating over a wide range of distances may be the key to explain the GRB
emission. In this picture, the peak of the spectrum forms at a Thomson optical
depth of several tens. The peak depends mainly on the bulk Lorentz factor Gamma
of the flow and can, therefore, be used to determine it. The Gamma is predicted
to range from ~10 to 1000 from X-ray flashes to the brightest observed GRBs in
agreement with recent observational inferences. The Amati relation can be
understood if the brightest bursts are the least baryon loaded ones.
Implications from this interpretation of the GRB emission for the central
engine are discussed.
