Hi! It's Jayden, and I'm back with another iMac problem lots of you guys have. So, you change your wallpaper, sign out, then sign in again, and your wallpaper is back to the usual mountain one.
If you have this problem on your iMac, then keep reading.
Go to "FINDER", at the left bottom corner of your desktop, look on the left bar, and click on "APPLICATIONS". Search "TERMINAL" at the search bar in the top right corner. Click on the app "TERMINAL", and a box with roboty writing will come up. Don't click anything there, just copy and paste this: rm ~/Library/Preferences/com.apple.Desktop.plist
After coping that, click "ENTER" once.
Log out, sign back in, and it will be your usual wallpaper. If you have any questions about weird things happening on your iMac, you're free to comment them down below.