I noticed a Texmod pack for a game using newly created normal maps, based on the color/diffuse texture.
The creator appears to have simply taken the diffuse map and converted it with something like NVIDIA Photoshop plugin into a normal map. Voila!
Well, if you take the time and look at the official normal map, those are completely different, containing blue to purple gradients, because those maps were generated by a program like 3DSMax from a high res mesh placed on top of the low polygon game model.
Obviously a Texmod retexture effort rarely involves creating a new, high resolution mesh too, because it would take a lot of time, energy that isn't paid.
In order to make your normal map generated from the diffuse map a little bit more apparent,
1. open it in photoshop and
2. duplicate the normal map layer
3. Change the top layers mode to Soft Light.
4. Duplicate this soft light layer to make the normal map more strong
5. When you are satisfied, Merge visible layers (to a new top layer, if you don't wish to collapse all layers)
6. Start the NVIDIA normal map Photoshop plugin
7. Select a normalize only and press OK
8. This will correct the color values so they are in the proper mathematical range.
9. Save this file as TGA for example and try it out in the game. You will experience much more emphasized details.
Fun fact:
Our developer company outsourced texturing jobs to Vietnam and China. We were particularly impressed with the Chinese colleagues: because they created an elaborate wooden door and achieved a "sculpted by hand" decoration effect via simply separating the normal map main colors and painted in sculptural details, like artists paint pictures on deviantart.