Ok i will try and make this fast.... lol
The Enigma could be the missing link between the Engineers and Xeno... we cant be sure their is some connection, the Frescos was not a easter Egg well i dont think they was, i feel they had a purpose to show us some connection....
What that is and why.... who knows.... this is one very very AMBIGIOUS part of the movie....
All i can try and do is connect the similarites between it and Gods be that Greek or the God of the Bible...
The First Fresco has a very Adam and God look to it from the Sistine Chapel...

If it is meant to be as such.... then The Engineer is God and the Creature is David..... so the Creature is the First Creation maybe they created on LV 223 in relation to the Xeno DNA......
But maybe this does not have to be the case.... The Engineers pose does not look very Powerful as that of God in the Adam and God Fresco... their is no link of God reaching out to his creation in this Fresco above..... it appears the Engineer is pushing away the Organism or is it petting it..... you decide...!
The creature has a pose that is either..
1) Fetal... as in born... created..
2) Submisive pose....
3) Pose similar to the Xeno Crab pose, like Fifield did a resting pose..
Is this a Fresco that shows the Creator (Engineer) and Creation... like the God and Adam Fresco....? Or what if we throw in Prometheus Myth.... is the Engineer now Prometheus? the one who Stole the Fire.... and is the creature the Eagle that was sent to Eat out the Engineers Liver.... as punishment?
Does the Engineer look inspired by the creature, or afraid.... again you decide because who knows?
There is some connection... there has to be.....
If we look at it in Terms of the Franchise.... we have the following...
1) Eggs => Face Huger => Chest Buster Organism... but a Chest Buster was created via Shaw and Holloway and the Go...... what i am getting at is how does this creature come to be with regards to Engineer.... is it the result of a Chest Buster Event?
2) Goo, we see this can mutate and evolve life it comes into contact with... again is this creature the result of Goo + Engineer? Who cant be sure..
3) Maybe there is no connection as above, but maybe this Organism is the Origin of the DNA that leads to those Eggs and also the Xeno related goings on in those Urns?
The 2nd Fresco...

This shows a Egg similar to the Xenos... it does not have to be a Xeno Egg but it has to be related... this is Evolved from or Evolves to the Xeno Eggs... It also does not have to be a Xeno holding it, because if you look close, the Creature in the other fresco has the same Arms and Hands...
I would safely say this is the same Organism.....
Why does it hold the Egg? some said its eating the Egg..... i dont buy that....
Maybe the creature is holding the Egg to be placed next to a Sacrifice, or for the Egg to play some part in a Sacrifice of its own DNA.... both these may imply this Organism can safely handle and transport the Eggs..?
But could this Organism also be Sacrificing itself to the Face Huger inside to create something? Again who knows...
Maybe it is consuming whats inside? But typically thats a Face Huger and as soon as the Egg opens the Face Huger latches onto a Victim Host.... i dont think its eating the Face Huger do you? Unless it does not contain a Face Huger?
Could it be actually laying a Face Huger... i.e does it produce a Organism that it implants via its mouth (i.e Spit or Vomits up) something that grows into a Face Huger... Again who knows?
What connection to the Sistine Chapel? i cant really make any... as the Frescos are depictions of events from the Bible..... maybe it is the Serpent offering the Forbiden Fruit?
If we look at Prometheus Myth... is it Fire? or maybe Pandoras Box? Maybe we can draw a connection between Forbiden Fruit and Pandoras Box...? Who knows?