I feel really stupid but I found an answer to my question BUT I don't know how to even follow the instructions. I know how to get to Finder but that's where it stops. I'm sorry but I don't understand. Can someone help me please and explain in differently? I understand that this is probably so simple but I feel like a dunce. Thank you in advance.
Below is copied from another post:
Diwu Oct 5, 2018 10:15 AM (in response to CoryT87)
This is how you can change your lock screen background (in macOS Mojave 10.14).
Navigate to '/Library/Desktop Pictures/' though your terminal/finder. There you'll find all your wallpapers provided by Apple. Now you'll find a file named 'Mojave.heic', this is the file that is used as your lockscreen background. Now you can make a custom background and save it as Mojave.jpg (make sure to use the correct dimentions). The last step is to change the extension to .heic. It'll ask you to use '.jpg' or '.heic'. Click on '.heic' ofcourse. Now log out and see the magic happen.
In short
1. Go to '/Library/Desktop\ Pictures/'
2. Rename custom image to 'Mojave'.***
3. Change extension to '.heic'