Wallpaper JPEGifier allows you to create JPG versions of LockBackground and HomeBackground.
As well as creating the unreadable .cpbitmap files when saving wallpaper, iOS will now create a .jpg at the same time.
The converted files can be found in /var/mobile/Library/SpringBoard/ with the same name as the .cpbitmap (with a .jpg extension of course). These files can be added to a lockscreen.
Please note: wallpaper set in the Photos app will not be converted; I can’t narrow down how they are being saved. Only wallpaper set in Settings will be converted.