Well, that's a bit tougher. My heightfield was graciously provided by Redrobes, et al. I haven't had much luck so far with adapting RobA's Gimp tutorials, but you might experiment and see what you can do with this one:
Making Height Fields in GIMP
Unfortunately, PS 7's clouds filter has no controls (frustrating), so I am not sure how easy it will be to adapt the tut. I don't know if later versions of Photoshop add controls to the filter or not.