Mural one: on the first trip to the Engineers dome-like structure the crew of the Prometheus encounter several murals/ designs that appear to have been created by the Engineers. The first mural shows a xenomorph-like figure in an almost Christ-like/ god-like pose, and the physical form depicted is not dissimilar to the alien that we see emerge from the dead Engineer at the very end of Prometheus. At the base of the mural is what appears to be an altar, which contains a glowing green crystal.
The key point that I took from this scene is that it conveys that the xenomorph, at least in the form shown here, existed and was known to the Engineers long before the events of Prometheus and Alien. Also, and perhaps most importantly, the style and positioning of the mural conveys that the Engineers hold this creature in high esteem- it's entirely plausible that they may even ascribe to it some sort of religious significance.
Mural two: the other mural appears to show an Engineer giving life to / creating something akin to a xenomorph hybrid. The layout of the mural is hauntingly reminiscent of Michelangelos Creation of Adam in the Vaticans Sistine Chapel. The subject of the creation lacks the grace and elegance of the original xenomorph in the first mural- it is stumped over and hunched and certainly different in style- which is why I term it a hybrid.
My theory: the first mural depicts a creature that the Engineers encountered in eons past. They revere the creature- viewing it in the same way that Ash talks about the titular antagonist of Alien - a perfect organismits structural perfection is matched only by its hostility..."
The Engineers are lovers of biotechnology and an organism that they considered genetically superior to themselves is clearly something that these lovers of genetic manipulation would revere and hold in awe. In their search across unimaginable distances of space and time this ancient true xenomorph may well have been the only type of life that the Engineers had discovered that they themselves had not created. The green crystal at the base contains the genome/ genetic remnants of this ancient creature, rather like the amber resin used in the movie Jurassic Park to re-create the dinosaurs.
Using these genetic remnants, the Engineers have been trying to replicate what they perceive to be the pinnacle of evolution and natural selection- they have not yet succeeded. The second mural serves to illustrate that the Engineers are indeed cosmic creators and superior genetic engineers: the exceptional genetic code of the original xenomorph has been used by them at length to further enhance their existing biotechnology, utilised in i) their bio-suits (which show xenomorph style markings and structures), ii) in their croissant-shaped ships (which appear to exhibit some bio-mechanical attributes), and iii) in their bio-weapons: most noticeably the black liquid...