The Texturing Portal
A texture is a 2D image wrapped around a 3D object. The word texture usually refers to the color map, which determines the object's color, but there are other texture maps that control the object's light reflection for example. A different texture can give a completely different look, many game objects use the same mesh but a different texture. Retexturing is a common way to give objects a unique look without changing the 3D model, but even for this changing the Nif file is necessary. The model is associated with a texture in the Nif file.
Creating detailed Normal Maps
You need to create a normal map first and foremost because if you don't, your texture wouldn't reflect light and therefore appear black ingame. What is more there is an even better reason why you want to do one. Normal maps are designed to make an object look much more detailed than it actually is. Based on illumination and the information stored in the file, a congrous surfase is simulated. In other words, you could stay in front of a completely flat plane with a stone texture that appears to be uneven and rocky, thanks to the normal map.
Not every texture has a normal map, only those that are rendered ingame such as tables, books, clothes, etc. Icons, book images, menus and whatnot aren't rendered ingame, hence they don't need one. The normal map's file name orientates itself by the texture, with the sole exception that _n is attached to it. E.g., the normal map for the texture Texture.dds would be Texture_n.dds.
One normal map can relate to an endless amount of textures. Example: You have a normal map and two textures. The normal map is called normal_n.dds, the two texture files normal_tex01.dds and normal_tex02.dds' respectively. The game will recognize both textures as belonging to the one normal_n; it simply ignores the letters that come after the underscores, in this case tex01 and tex02. So you may want to avoid texture file names like my_uber_sword_texture.dds.
And why is the normal map's detail important? Because it is the normal map that defines how uneven and how rocky your stone texture looks ingame. Less detail for the normal map equals less detail for the object.
Things you can do
Using Images is often the most effective way of texturing. Try it