AppleScript Primer for Mac OS X
Subject:   Desktop picture?
Date:   2004-02-16 08:55:21
From:   jaharmi
Response to: Desktop picture?

You can fairly easily set the Finder's desktop picture this just so:

tell application "Finder"
set the desktop picture to document file "Tiles Warm Grey.jpg" of folder "Desktop Pictures" of folder "Library" of the startup disk
end tell

I don't know of a way to script setting the background picture for a Finder window, yet, as of Panther.