Anne, I did some research in another forum and this is what I found. You will need to edit the
"LockBackground.html" via ssh or ifile. If you want do do it via ssh you can download WinSCP to do it. ifile you can get via Cydia and they have a trial period.
You need to change the following
From:
<div id="background">
<img style="position:absolute" src="background.png" width="320" height="480"></img>
<img src="
/User/Library/SpringBoard/LockBackground.jpg" width="320" height="480"></img>
</div>
to:
<div id="background">
<img style="position:absolute" src="background.png" width="320" height="480"></img>
<img src="
<your wallpaper name.extension>" width="320" height="480"></img>
</div>
In my case I replaced it with
LockBackground.png and it worked.
Good Luck!