How to make desktop pic center, stays in fit to screen

Create another admin user account for troubleshooting purposes: Set up users, guests, and groups on Mac. It takes up very little space and is good to troubleshoot problems.

I had better work on honing up my mind reading skills:

What size is the image? Works fine here.

That's a different problem. Does it work in a different user account?

I suspect it's a corrupt plist. Unfortunately I don't know which one. Hopefully someone else will and can assist.

FWIW, you shouldn't create new posts for the same problem. I saw you don't have the problem in a new account which means it is a plist problem. Someone has to figure out which.

You can see if deleting this file fixes your problem. Be aware you may have to reset any system preferences you have changed.

~/Library/Preferences/com.apple.systempreferences.plist

FWIW, the OP had already created a new user and the problem didn't occur. There's no way you would have known that since the second post was deleted.

😎