you could maybe use an extra megapixel, but this is a beautiful picture. nice choice of angle on the center boulder, it's like a waning moon the way the light hits it.
I loved the Devil's Marbles... what a beautiful sight they were just as the sun was going down :-) I think this would have filled the frame more, and been better maybe as a horizontal picture with less in the foreground and less in the background... filling the frame would have been nice. Also the main focus here is very centered which is kind of boring. Love that place though :-) OZ... I mmiss OZ!
Is this the name of the place where you shot? Nice subject, good focus and colours. I like that the is the single 'marble' as the main subject - nicely framed. Slightly grainy but still a very nice picture. Good luck
Good idea for round, the horizon line may have worked better in the upper or lower third, it could be a tiny bit sharper, and the banding noise in the sky is a bit distracting, but still a nice photo.