Without having 100% yet and just throwing my 2 cents out there for anyone...
Before i begin i also wanted to point out that the symbol atop chiliad, under the telescopes, that symbol only has 1 dot below it, seems most are overlooking this and assuming it has rain under it.
Anyways, my thoughts are that the mural shows you an order in which to see/interact with the UFO's to unlock whatever item is on the left or right side, and unlocking both sides(UFO/Jetpack) unlocks the main easter egg, whatever that could be.
also, the symbols on the sides of the mountain have all been white and from what i can tell the symbols found elswhere are red/yellow, is this correct?
Assuming that it is i see that the symbol atop Chiliad is RED and we also have RED symbol of UFO/Eye+Rain at the hippy camp. So red matches w/red and you get a UFO during the rain atop Chiliad.
So start at Chiliad UFO and see that one to start i'm assuming.
The next symbol down the right side of the mountain/mural is the "Beam" symbol, nobody to my knowledge has found a doppleganger symbol for this one anywhere on the map so it throws a curve into my theory BUT, this looks to me like it is representing the UFO at the Army Base that is shining a green light down. So i guess you would go see/interact w/this one next.
On to the next symbol down the right side of the mountain/mural, the UFO over/under the Moon symbol, this symbol is YELLOW(no idea why, yet). We have a duplicate of this symbol at the Hippy camp also, which is where i got the idea of going to the same spots in different orders, it seems you need to come see this UFO 2 times, maybe 2 different orders for 2 different easter eggs?(UFO egg + Jetpack egg?)
This is kind of where i'm getting lost since im not 100% yet.
another thought is the lightning bolts on the mural represent different times, like a clock, and you need to go see certain UFO's in order, clockwise from bottom left to bottom right?
Seeing as how EVERYONE said "Jetpack" when asked what they wanted from GTAV, i dont see why they would not include one, it's what the whole internet was talking about, if they didn't include one the super big time fail on R-Stars part(but would they make it so difficult to obtain of every1 wanted a JP?)
Sorry for the typos if there are any, it's late.