One more time...
I open the file walker.def because I want to examine the entity declaration and see what model is used for the walker entity and possibly the name of any skins applied. I find that the model is called model_vehicle_walker and there is no skin key/value pair so skins are not a concern.
The model declaration points to an md5mesh at models/vehicles/walker/walker_base.md5mesh so I look to this file to gather what material shader is being referenced. I find quite a few shaders...
I assume you want to skin the walker itself so I'm going to run with the shader models/vehicles/walker/body because it's name is a tipoff.
So, upon examining this material shader I find it references the images...
models/vehicles/walker/body_d.tga is the diffuse image and is probably the one you're interested in.
Now, because Doom 3 engine based games use Targa images for High end settings and DDS images for Low end settings, it's critical that you replace both the TGA and DDS version of this image in order for the change to take effect irrespective of video settings.
So your files should be named and stored as ...
If any of this information is new to you I highly suggest you read over the texturing section on http://www.modwiki.net
because all of this is covered there.
On a side note, it's bad practice to override the game's stock assets and work from within the base directory. It's much more professional to work from within a custom mod folder and create new assets altogether.
It's a relatively simple matter to write the necessary skin declaration, material shader, and entity declaration so that you have an additional version of the walker listed from within the level editor.
But I'm not going to elaborate on the details here. At least not at this point because you have quite a bit of information to absorb as it is.