A good way to make bump maps if your using photoshop is to use the lighting effects filter.
Set the style to parallel directional, light type to directional, and adjust to get the effect you want. Set texture channel to blue or green and adjust height- flat to mountanous to get the intensity of the map you desire. 
You must use an image with color info for this filter to work, then convert to greyscale after you have acheived the results you want. This method requires a little experimentation to get just what you are wanting. Sometimes if you are wanting are very subtle effect just a plain greyscale image will work.