I thought I'd share a couple of pictures which highlight the differences between injecting liquid versus gaseous or saturated propellants into a combustion chamber.
This first picture is of a single injection orifice flowing water from my garden hose - the stream is very laminar with very little dispersion. Engines with liquid injection usually use impinging streams to accomplish the mixing that is required for good combustion stability and efficiency.
Robert Watzlavick has several good pictures of an impinging liquid-liquid injector on his website, in addition to this one.
The last picture is of the same injector in picture number one flowing liquid N2O instead of water. In this case, the saturated liquid flashes to vapor immediately due to the much lower atmospheric pressure. The resulting plume is easily 10 times the diameter of the injection orifice. Gaseous injection would probably not result in such a large plume but would still expand much more than the water above.