I found this blog post titled: How to Change the Login Screen Background In macOS Sierra, Yosemite, and El Capitan which illustrates a method where you first select the Change Background option by double tapping on the background.
After doing so go through and change your background to whatever image you'd like it to be. After doing this if you look through your system's
/Library/Caches you should see a blurred version of your background as a
.png file there.
$ sudo find /Library/Caches/ |grep png
If you change this file out with another file that you'd like to use for your lockscreen you can then override the blurred version that macOS automatically generated for you when setting your background.
NOTE: Just be sure to keep the name of the file identical to what macOS placed in your