Steam\steamapps\common\left 4 dead\left4dead

You will be modifying the files in this folder, if in the event you're unsatisfied with the background you've chosen, or if modifying these files is crashing/breaking your game. Replacing the folder with a backup is the quickest solution to a fix. Again this is optional, but I highly recommend it.

The current official background Left 4 Dead uses is "l4d_crashcourse_bg". If you wanna select 1 of the other 2 backgrounds, all you have to do is this.

-Look at the previews below.
-If you found a background you want to use, open L4D's media folder.
-Delete or Rename the file called "l4d_crashcourse_bg.bik"
-Locate the background you want to use.
-Rename it as "l4d_crashcourse_bg"
-That's it! Launch Left 4 Dead and enjoy your new background.