The tile IS a problem, and I believe it's a "Select Ground" marker, which can easily be removed.. >.>
The rocks looks fine, as does the grass, really, COULD have some more variation though, but that isn't the largest problem. The problem is that the tree on the right side that is severly more green than anything else doesn't fit in, I would suggest removing it and have something else there. Second, is the fog. The fog and the sky doesn't work together, as it is now the fog makes the sky seem like a "The End" frame on the far horizon, while if you had a darker fog, and somewhat denser, it would seem like your terrain is fading away, making a nice looking effect.