  The formation of complex organic molecules (COMs) in protostellar
environments is a hotly debated topic. In particular, the relative importance
of the gas phase processes as compared to a direct formation of COMs on the
dust grain surfaces is so far unknown. We report here the first high-resolution
images of acetaldehyde (CH$_3$CHO) emission towards the chemically rich
protostellar shock L1157-B1, obtained at 2 mm with the IRAM Plateau de Bure
interferometer. Six blueshifted CH$_3$CHO lines with $E_{\rm u}$ = 26-35 K have
been detected. The acetaldehyde spatial distribution follows the young ($\sim$
2000 yr) outflow cavity produced by the impact of the jet with the ambient
medium, indicating that this COM is closely associated with the region enriched
by iced species evaporated from dust mantles and released into the gas phase. A
high CH$_3$CHO relative abundance, 2-3 $\times$ 10$^{-8}$, is inferred,
similarly to what found in hot-corinos. Astrochemical modelling indicates that
gas phase reactions can produce the observed quantity of acetaldehyde only if a
large fraction of carbon, of the order of 0.1%, is locked into iced
hydrocarbons.
