It can be more complex than simply creating a greyscale. A good bumpmap needs to follow a topographic scale that highlights convex topography, while undertoning concave qualities in the texture. The bumpmap does not necessarily need to follow the exact color patterns in the working texture.

Something truly nifty with some software, is the ability to apply the bumpmap to the actual modeled mesh and have the model pop into true 3d shape at a simple click. This is possible with bryce terrains and lattice meshes, which can be much more realistic when render and lighting effects are finally utilized.

This example below, shows a cushioned armchair in it. This chair was created with a bumpmap, which when applied to mountains and clicked, makes an instant armchair from a terrain mesh. Actually the picture frame above the chair, the books, the lamp base and the bumpy tiled floor are all made with grey scales from Photoshop and then applied to the models themselves. Not applied as a texture bump map at all, which is also an option in Bryce.