Desktop wallpaper reverts after restart

I'm experiencing the same thing on a Mac Pro. The thing is though, I had changed the desktop background and when I go to Preferences > Desktop & Screen Saver, it SHOWS the wallpaper I want previewed in the thumbnail however, the ACTUAL background defaults to Mac OS Sierra.

FOUND A WEIRD FIX: With the Desktop & Screen Saver panel open, there are two drop down menu's to the left for me. One says 'Apple' which contains a folder named Desktop Pictures and Solid Colors. And below that there's a Dropdown Menu Named 'Folders' which contains a folder named 'Pictures'.

Once I clicked on the 'Pictures' folder, my background immediately and magically updated to my desired image (which again, was already loaded in the preview thumbnail above earlier). I also have an extended monitor where the image was defaulted to Mac OS Sierra, I did this same thing, and the default Mac OS image changed to what I wanted it to as well. Weird fix, hopefully this helps you too!

Start up in safe mode

To start up in safe mode:

1. Start or restart your Mac, then immediately press and hold the Shift key. The white Apple logo appears on your display. If you don't see the Apple logo, learn what to do.
2. Release the shift key when you see the login window. If your startup disk is encrypted with FileVault, you might be asked to log in twice: once to unlock the startup disk, and again to log in to the Finder.

To leave safe mode, restart your Mac without pressing any keys during startup.

Try safe mode if your Mac doesn‘t finish starting up

If everything is working properly in safe mode, but then is not working after you restart the Mac and boot back into your user account, please check the section titled "If an issue doesn't happen in safe mode" for troubleshooting steps. If you see the behavior persist, follow the steps below to test in a new user account. Testing in a new user will show if what you're experiencing is related to your user profile exclusively.

How to test with another user account

You can figure out if unexpected behavior is related to user file or setting by trying to reproduce the issue from another user account. This process includes creating a new user account, logging in to it, and testing for the issue.

Create a test user account

1. From the Apple menu, choose System Preferences.
2. Click the Users & Groups icon in the System Preferences window.
3. Click the lock icon, then enter an administrator name and password.
4. Click the Add button (+) below the list of users.
5. Choose a type of user from the New Account pop-up menu.
6. Give the user a full name, account name, and password.
7. Click OK.
8. Close the System Preferences window.

If there are documents from your original user account that you want to test with, place a copy of these items in the Shared folder in the Users folder. Press the Option key while dragging a file to this folder to create a copy.

Log in to the test user account

Log out of your current user account by choosing Log Out from the Apple menu, then log in to the new account you created. If you're prompted to sign in with your iCloud account or Apple ID when you log in, skip this step.

Try reproducing the issue

Try the same steps that caused the unexpected behavior to appear before. For example, if you were unable to print, try printing from this user account. If you were unable to connect to the Internet, try browsing a website from this user account.

If you were using any specific settings that testing depends on (such as using a specific email account or iCloud account) set up the same account in the test user. For email and most other settings you can use the Internet Accounts pane in System Preferences to set up these kinds of accounts. If files from your home folder are needed for testing, copy them to Shared folder in the Users folder (/Users/Shared). After logging in as the test user, copy these items to the same location in the test user's home folder to test with.

How to test an issue in another user account on your Mac

If the issue is being experienced in the test user also, then we need to check the section titled "If an issue happens in more than one user account" from the article provided above which will lead you to this help article: Mac OS X: How to troubleshoot a software issue.