Edit this file (you need root access
and add the key as below
"homescreen-wallpaper" = 1;
The default HomeBackground.jpg and LockBackground.jpg are here
/Users/yourusername/Library/Application Support/iPhone Simulator/4.0/Library/SpringBoard/
As a bonus, if you add this homescreen-wallpaper key-pair, the problem of "iPhone simulator goes black when trying to taking app to background" problem will be solved.