When dealing with Graphics used in DoD:S or other HL2 games for that matter, the file you will see quite a lot of is a .VTF file.

It's basically an image format used by VALVe that can store/use an image in a variety of different ways. It's a bit of a read but see this page (come back to it latter if you like) for a more detailed description of the VTF format.

I won't kid you... this is not easy... but stick with it and when over the learning... the fun begins :)

The Tools you will need to customize a VTF.

GCFScape. Free... from Nems tools.
VTF Shell Extensions. Free... Neil 'Jed' Jedrzejewski. (mirror link)
A Graphics Program that handles the "Alpha" channel... Free or commercial?
The GIMP and VTFEdit. Free... Neil "Jed" Jedrzejewski and Ryan "Nemesis" Gregg.

Recommended for Windows...

Paint.Net and the VTF Plugin (from the internet achieve) for it, both free!

VTF Shell Extensions.

Making Windows display a VTF file in Thumbnail view...


vtf_thumb Setting your Folder view on "Thumbnails"
makes it easy to explore a folder full of graphics.
You can do this for VTF files but you will
have to install some files.
 See this page for more details.

Download from the mirror site. here. 

Jed has put in a lot of effort into DoD and Half Life over the years. The latest project (now stopped) he was working on with others is a WW2 HL2 Mod Ham and Jam. 

VTFEdit

This is a handy tool for users of graphics programs that can not use Nems VTF Plug-In (for Adobe Photoshop 6.0 and up).

VTFEdit is a graphical frontend for VTFLib. This nice little tool is the result of two great contributors to the Half Life 2 custom community...  Neil "Jed" Jedrzejewski and Ryan "Nemesis" Gregg.

The information and some comments are here. The download page is here.

NOTE: You will also need the .NET Framework!

What this tool will do for you is convert the VTF file into a format the graphics program can edit and then save it as a VTF file.

So users of the Gimp or others like it ... this is a must have program!

There is already quite a bit of documentation on this tool and rather than repeat it I will just provide some links.... 

A basic outline of the tool  The Valve community Wiki

Description of VTFLib  VTFLib