One way that may require the help of an octopus is to go to Wallpaper settings click on wallpaper preview that is not the lock screen and pinch the image so it shrinks in the screen - this will get the "Cancel/Set" buttons out of the image. If you let go of the pinch, it'll bounce back, so the trick is to pinch and screen grab at the same time. Then you can crop out the wallpaper setting details in newly captured image in Photos. I was able to do this with four fingers on two hands (no feet required :)) on an iPhone 4s, not sure how dexterous you need to be on an iPhone 6 or 6 plus.