Well, Justin, it often depends. With a brick wall and light mortar, you would create a negative but when the kmortar is darker, you can leave it like that.
Actually to create a really good bumpmap, you need to tweak that image a lot more. What Remus posted, has darker and lighter areas and the wood board bumped with it would seem not to be well sown at the minimum. But basically this is the method.
If you wish, you can Google for "Crazy Bump" which is a beta tool for generating and tweaking bumpmaps (and specular maps and displacement maps and all those funky ones not even mentioned here).
Also, the folks at the Podium forums have shared a bunch of them as well as have a nice collection of material site links (like we do, too) some of which contains bump and/or specular maps with the texture diffuse maps. Look around a little bit and you will find a lot I bet.