For those that want the background of the lockscreen to be defined on the phone through the Wallpaper Menu, and to able to change it without having to ssh in new backgrounds and rename things and what not, change your LockBackground.html to this under the <style> section (obviously properly indented and all:
Code:background-image: url(/private/var/mobile/Library/SpringBoard/LockBackground.jpg);
Now when you change your lockscreen background in Settings -> General -> Wallpaper, the lockscreen background will change. (Note: Requires a respring for the wallpaper to change, i'm trying to find a way to get it to refresh when you wake the phone, don't know if its possible.
My next thing is to work out how to display new SMS/Missed Calls/New Email on the lockscreen using HTML/css/js.... anyone want to point me in the right direction?
i've tried changing background url to your tip but i can't seem to get it to work. i still get a black bg even after respring. anyone having the same issue?