It's not like raz's theory is flawed, I think dips and bumps are the idea.

Anyhow, if you're unsatisfied with the level of Dip, or Bump, then you can open Photoshop's Image > Adjust > Levels dialog.  The two lower sliders will control the black and white extremeties.  In other words if you move the left hand side slider toward the middle, then your image will no longer have %100 black pixels, rather it's darkest value will be a shade of grey.  You can also stop an image from having %100 white pixels, with the right hand side slider, in the same fashion.