  The prompt emission of gamma-ray bursts probably comes from a highly
relativistic wind which converts part of its kinetic energy into radiation via
the formation of shocks within the wind itself. Such "internal shocks" can
occur if the wind is generated with a highly non uniform distribution of the
Lorentz factor. We estimate the expected photospheric emission of such a wind
when it becomes transparent. We compare this thermal emission (temporal profile
+ spectrum) to the non-thermal emission produced by the internal shocks. In
most cases, we predict a rather bright thermal emission that should already
have been detected. This favors acceleration mechanisms for the wind where the
initial energy input is under magnetic rather than thermal form. Such scenarios
can produce thermal X-ray precursors comparable to those observed by GINGA and
WATCH/GRANAT.
