I'm tinkering around with a map for a post-apo/sci-fi type setting. Now I'm new at this whole mapping business and I'm using GIMP, which I'm also new at. However I'm having a ton of fun learning. My main problem atm is how to make a background for the map. Like on many fantasy style maps you'd have grassy type background. Well in my case I want a ruined city! Problem is, I have no idea on how to accomplish this.

I've read some tutorials on rivers and mountains and all that hawt stuff. So far adding in the various spots on the map such as those mentioned above, have been fairly easy and with some tinkering I'm sure it'll blend in well. But the background I have absolutely no idea how to make. Any ideas or suggestions perhaps? I'd be extremely grateful. And I do hope I posted this in the right forum, seemed the most appropriate one to me.