well, if you zoom into that it's not too complicated..
Sure you can make it better, but i think it's easier to get it to tile when it's simple..
And, i was actually suprised i did that, because normally when i try rocks they come out terrible =P
Hope it helps :]
If you want some more help you can always add me on msn, if you have it. It's on my profile or if it's not, then pm and ask me :]
-Quake