It's no masterpiece, believe me! (If I'm being honest, I don't think it should have won...)
But I'm able to do something like this just because I've gotten used to using GIMP. Lots of color picking, zooming in, etc. You've just got to get used to doing all the steps as quickly as you can!
The rocks aren't beautifully shaded or anything, but playing around with little details like that helps as well. You can do something like this! Just practice.