Texture map is different than height map. Height maps are displacement maps. Greyscale (you can have colors, but only the RGB intensity information is used). White is highest, black is lowest. That doesn't neccesarily coincide with the coloration of a photograph. Just because is white in a picture doesn't mean that it would be the highest point, if you see what I mean.
You normally have to use a 2d editor to paint over the photograph; if you want to derive a displacement map from a photograph.
I almost always use DEMs for terrains, when I need real world terrain info. That is what a DEM is for. Otherwise I just make them with noise functions and/or paint them.

EDIT - Bryce has a lot of fractals to use for terrains, and of course the terrain brush...

Friends don't let friends use booleans.