The photographs show a television displaying a segment of the Los Santos map, overlayed with Leonardo Da Vinci's Vitruvian Man.
The photographs were allegedly published by a Rockstar Games Social Club user named Quantum Anguli, which is Latin for as far as the corners.
Quantum, as defined by Google, means a discrete quantity of energy proportional in magnitude to the frequency of the radiation it represents. Thus meaning that the corners may have something to do with energy, radiation, or nothing at all.
The myth itself is very vague and so means it would be very hard to follow, however, many investigations have searched for The Quantum Man. No proof supporting this myth was found, but nobody quite knows exactly what they're looking for. But it could be a spiritual barrier of some kind blocking off the outside of the map.