As for a tutorial on how to modify an image:
1) Extract the resources with folder (either all with my extractpak.exe or some specific files with Serges tool) in your game folder.
2) Find the dds you want to modify and load it in your favorite image editor that supports dds. (XnView can convert dds, there is a plugin for Photoshop and you could also use the DirectX Texture Tool from the DX SDK, etc..)
3) Convert the dds back to dxt with "dds2dxt.exe xxx.dds xxx.dxt" - remember to put/leave the dxt in the original path.
4) Launch the game and test it.
5) Play barber and go to 2)