Friends Of Dirty

Try this: (courtesy of the L4D forums)

Navigate to the following folder

Steam\SteamApps\common\left 4 dead\left4dead\media

For restoration purposes, make copies of l4d_background.bik and l4d_lighthouse_bg01.bik

Rename l4d_lighthouse_bg01.bik to something else
(I renamed the copies and put them in a "Backups" folder within the media folder.)

Change the name of l4d_background.bik to l4d_lighthouse_bg01.bik.

Your menu is now the original, enjoy.