always a way to configure...
if you can use ifile, you can go into theme and remove lockbackground.png
or in winterboard you can put user lock background at the very top of the winterbaord list (i dont know if that feature still works though)
iFile is like an file explorer program. U have to locate the LS theme file and then locate the .png file it uses for the Wallpaper. Just rename whatever wallpaper u want to use to that name and respring and it will update to it wallpaper. Good luck. DO NOT change any other files or names or u can mess up the theme