If you have artistic skills (illustration or painting) then you could do it in Photoshop, Paint Shop, Painter, etc. but if you don't have those skills then you might want to look into 3d programs. There are a few programs that are free that can do up some random terrain and you can set the camera angle. In order to get trees and stuff you might have to look into buying some software and there are tons. The one that I really want to look into I can't remember the name of...something like Nature 3d or Landscaper or something like that...its main function was not 3d cars or spaceships but trees. If I can find the link I'll post it up...I just remember that I was searching for 3d trees on Google once and found it.