On the back says: 'Keep or Rehide' 'WPG Rocks'. Just found there is a fcebook group, artists hide them to be discovered. Not an English speaker so I am not sure if that 'keeps' means that I could bring it with me, so I rehide it to be found by another person :)
(Ugh I commented on the post not comment)
Sorry, let me expand. The poster asked if it was really littering if it was a rock(but painted). I meant to use graffiti on a wall as a similar example. Unpainted rock=unpainted wall. Painted rock=graffiti.