This article sums it up pretty well. (Credit to Cresh for the find)
Thanks to the sleuths at IGN, the message has been deciphered. It reads:
SUN BIRD BULL LION SUN
If you stand near each of symbols on the ground that correspond to these words in the area around the painting and press R1, the painting changes to a different message, which reads:
When the earth stops, the journey begins...
The most obvious meaning behind this is a reference to the next game, but that may not be true.
Some have theorized that the words "Sun, bird, etc" are referencing Egyptian mythology, which may hint at the next game taking place in Egypt.