When you type "nano .config/MohammadAG/enhanced-lockscreen.conf", this will create a file called enhanced-lockscreen.conf. You should be in your home directory (/home/user/) when you type this command. Instead of nano you can use leafpad which worked fine for me.
An empty document (newly created and named enhanced-lockscreen.conf) will open. In there you type:

[main]
background-url=/correctpathtothepictureyouwant/

And then you save, type "killall enhanced-lockscreen" as root. And then you type "enhanced-lockscreen". Then the image should appear

Remember that the pathway in the conf-file should be from the root directory. E.g "/home/user/MyDocs/.images/picture.jpg" if your picture is stored in the pictures folder

Sorry for the long reply!