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.
This tweak uses MobileSubstrate by saurik, and was created with the help of theos and Logos by DHowett.