This method works for quite a few system images, not just the lockscreen. The problem is knowing which images to change in which folders. I would love to have some kind of annotated list from a handy website. I found something similar for editing Windows resource files where everything was numbered and mapped for convenience, but I haven't found anything like it for Android. Does it exist?
Note to OP: Nice and simple write-up - thanks! - but GIMP should work just as well as Photoshop (for us free-loaders)