The lighter the color, the higher it thinks the pixel is.
lookup some bumpmaps to see examples.
I'm not an artist, so I can't tell you the best way to make them, but here's my recommendation:
Manually draw the bumpmap. See how their done, and draw one to match your image. Then add some noise if you can. And then use a SLIGHT gausan blur to smooth it a bit. Then convert it with your NMG and there you go!
You can try a grayscale image of the original texture as well, and it will work, but it will not be correct and look bad.