The latest version of s4pe now unpacks and previews these images, which should make it easier to find what you want. Turns out they're not normal maps, they're a custom format with pairs of images which the game combines into one complete image. I'm currently working on a tool to convert them to/from one image - got the combination into one bmp working and still have to do the conversion back to two files.
Please do not PM me with mod, tutorial, or general modding questions or problems; post them in the thread for the mod or tutorial or post them in the appropriate forum.
Visit my blogs for other Sims content:
Online Sims - general mods for Sims 3
Offline Sims - adult mods for Sims 3 and Sims 4