I'm still curious about the relief of the xenomorph on the wall myself. If they're just supposed to be another bio-weapon or black goo mutation, why are they the only ones that share the same Giger-esque ribbing and bio-mechanical look as the Engineers?
In the "Art of" book, Ridley talks about the mutations taking on the form of their hosts, but it was shown pretty clearly in the movie that the bio-mechanical detail was only part of a bio-suit, and the Engineers look much more human underneath. So I don't think the xenomorphs could have gotten it that way.
Maybe they were some kind of special creation the Engineers patterned after themselves or something. Or maybe they even come from the same home planet...
Was wondering that myself. It is possible that, in order to successfully sacrifice themselves for the creation of new life via the black goo, they had to purge some inner demon/all negative energy. Perhaps the mural of the proto-xenomorph is a warning to the Engineers, what could happen if any negative energy remained within them and attempted to use the black goo without being properly prepared. The facehugger/squid thing that came out of Shaw definitely had the same pointy-headed look to it that the proto-Alien had (matching the mural), so it could already be considered corrupt. The corrupt nature of humans had infected Holloway's "seed" and was further corrupted when gestating within Shaw. The squid further carried that corruption to the Engineer at the end who, himself, may have also been one of the corrupted "fallen angels" of that race that wanted to see our entire species extinguished. What we see emerge from his guts is the result of that collective corruption and negative energy, as predicted by the mural.