This image is found at the top of Mt. Chiliad, painted on the wall of the lift building, next to what appears to be a metal door.
My first thought when I saw this was that it was, quite obviously, a map. This map appeared to lead to 3 things: a UFO, a cracked egg, and a jetpack.
A lot has been speculated regarding the meaning(s) of this mural. It could be a physical map, leading us to physical prizes (ie: flyable UFO, jetpack, dinosaur w/saddle), or it could be metaphorical.
So far, there are five glyphs, which have been found around Mt. Chiliad, that seem to match up with the 5 small boxes on the mural. There is also a red "eye" located under the viewing platform at the top of Mt. Chiliad, which matches the image at the top of the mural.
This is one of the glyphs that were found on/around Mt. Chiliad, referred to as "cs1_10_clue_mountain01" in the game's files. There have been many different ideas regarding this glyph, and what it may depict. According to the filename, it appears to be a mountain with a trail leading to where you can see "the eye/UFO". However, it has been speculated that it may also be a UFO with a tractor beam, or a lighthouse, among other things. Unfortunately, there is really no way to be certain, short of Rockstar coming out and telling us exactly what it means, and I highly doubt that will happen any time soon.
Here is another of the glyphs that were found, referred to as "cs1_10_clue_moon01" in the files.This glyph seems to represent 3am, which happens to be time condition necessary to view the Mt. Chiliad UFO.
A third glyph, found on Mt. Chiliad, which clearly signifies rain. The filename for this is "cs1_10_clue_rain01". This is the second, of three conditions, necessary to view the Mt. Chiliad UFO.
This glyph is referred to as "cs1_10_clue_rain02". Not much is known about the meaning of it... It seems to simply be a faded/worn "eye" or UFO...
This glyph is referred to as "cs1_10_clue_moon02". Much like the "rain02" glyph, not much is known about the meaning of it. It appears to be a half "eye"/UFO, tilted sideways...
So... We have a mural which appears to show a mountain, with 5 small boxes, and paths leading through said boxes, to 3 large boxes containing a UFO, Cracked Egg, and Man w/Jetpack. We also have 5 glyphs which have been located on/around Mt. Chiliad. One final clue is needed to see the Mt. Chiliad UFO...
Here you can see the writing on the back of the Mt. Chiliad viewing platform, which tells you to come back when your story is complete. Underneath the platform, you can also see the RedEye glyph, assumed to signify that this is where you will get your reward/Easter Egg.