Ok, time to show you how to make normal maps.
What the hell is a normal map?
A normal map is a RGB map that indicates to the engine the normals of each pixel. Red, Green and Blue indicates the X, Y and Z values to the engine. This give your model more details without adding polygons and using low resources. Note that in Half-Life 2, DX7 has no normal mapping at all, DX8 only on world, and DX9 on both world and models.
How do I make normal maps?
There's 2 ways to make normal maps. The first one uses a higher poly model and a plug-in in your 3D application. The second, which I prefer, only requires Photoshop. I'm going to show you the second way to make normal maps, since using a 3D application sometimes differs from a program to another.
First step is to download the NVIDIA Normal Map plug-in here. Install it and open Photoshop. Then load your texture you want a normal map for. Create a new layer over it.
To use the plug-in, you need to create a greyscale map of the height of your texture. More its white, higher it is. Darker will make a lower bump. Here's an example map. Look at the render below it, you'll see the dark parts made down bumps and the white parts made higher bumps.
GRAYSCALE MAP + NVIDIA NORMAL MAP PLUGIN
The most common technique used on complex texture is to put the texture itself in greyscale using Desaturate function in Photoshop. But I don't recommend this technique since the colour of a texture does not always represents it's bump properties.
Hope this helps a bit.