The Source folder contains the files used in developing the final released files. These files are in formats for easier editing and need to be transformed or compiled into the file formats needed by L4D2.
Create the Source folder:
Within the "workspace" folder, create the development folder: 02 Source
Note the "02" at the start of the name. I use that so the workspace folders are listed in this order: 01 Setup, 02 Source, 03 Release
Within the Source folder:
Create a folder for image file development: images