VTFEditor - An application that allows you Open (VTF/VTM) Files! Wich are Textures or Codes so you can edit!
But whats really cool , is that you can use this app for making Animated Gif Sprays! (Not only for L4D2 but also Css or Cs1.6/Cz)

So lets say you are making a Simple Pistol Retexture alright?
Inside pistols folder , there are 3 filles and 2 of them are textures , you can edit one of them only (if you want to edit something in codes , you can do that as well)
(Save it as BMP , saving it as TGA or PNG will make Textures Invisible for some reson)

(You can edit Textures with WHATEVER photoshop. No matter what photoshop it is. You can even use gimp just like i do in most of my mods!)