Where can I locate the picture that is currently being used for my lock screen?
From what I can tell from the other answers it sounds like the image you wish to retrieve is also your wallpaper.
If that is the case you could use an app I have been working on for this exact purpose, called Get Current Wallpaper.

It will retrieve your current wallpaper, save it to your Pictures folder on your external storage drive and allow you to share it with whatever app you use to share images :)