Making a really good CSS sprite image usually isn’t trivial work. There are all kinds of tools to help with it. My favorite of them has been SpriteMe. I describe my typical workflow for using that here. Sprite Cow might be my new favorite though, can’t wait to try it. With it, you design your own sprite (♥) and you use the sweet Sprite cow interface to get precise CSS background positions for the different parts of it.
Sprite Cow