I also cannot change the lock screen image.
On Setting > Screen lock > Screen lock style -
there is only one 'option': "Magazine unlock" with a generic purple image.
I did the reset suggested on this page but it didn't help.
I have no power-saving stuff. Advice from LG support staff was conflicting, but then one of them said LG doesn't allow any app except native 'Messaging' to be default because then users might be able to (e.g.) send more text messages than their contractual allowance...! Seems like a really weak...