The Engineers wear large Grey/Silver Pressure suits when in combat or when doing any sorts of physical activities, they suits consists of a large helmet with a "Trunk" like feature in the middle connecting to the chest, this may be for breathing. The suit has tinted lenses for seeing, the suit also looks like some sort of armor due to the hard padding and layers of a metal like material.
The Engineers created the human race as you see at the start of Prometheus where the engineer sacrifices himself by drinking a black fluid which makes his limbs fall of, this spreads The Engineers DNA in water and various micro organisms, effectively creating the human race, this is why they look so much like humans as they were created from the Engineers' DNA, making Engineers ancestors of the human race, as Dr.Shaw predicted. The Engineers did this as they were leaving Earth for unknown reasons.
As the crew of the SS Prometheus found, the Engineers had ships full of a strange black liquid, later known to create the race of the Xenomorphs, But why would they create this? And why would they send this to Earth?. The black liquid is the origin of the Xenomorphs, but first the worm-like creatures must react with it, creating the Trilobite also known as "Facehuggers" which latch on the face of an enemy impregnating the enemy with the Xenomorph, creating the alien race.The cases of the black liquid were set to leak/open at a atmosphere the same or similar to Earths atmosphere, Was this due to war? or Genocide of the Human race?
So what do the cave paintings mean? Dr.Shaw presumes that its an invitation, this may mean that the Engineers were friendly with the Humans, or if they were at war with the humans the paintings could be a map for the future, when humans could match the advanced technology of the Engineers.