Get a copy of iphone lock screen
I wanted to keep a background from an older version of iOS that was no longer available.  I did the following.


Take a screenshot of your lock screen by pressing the Lock and Home buttons simultaneously.
Get the screenshot onto a computer by email or syncing.
Go to Google Image Search, press the camera icon, and upload the screenshot.
Find a suitable copy to download.


Unfortunately, if it's a photo you took, this won't show any results unless you've also uploaded the photo to the internet at some point.