Mac wallpaper Changes by Itself to default

Give this a try: boot into Safe Mode according to How to use safe mode on your Mac and set the wallpaper to what you want. Reboot normally and see if the changes stick or the problem returns.

NOTE 1: Safe Mode boot can take up to 3 - 5 minutes as it's doing the following; 

• Verifies your startup disk and attempts to repair directory issues, if needed

• Loads only required kernel extensions (prevents 3rd party kernel/extensions from loading)

• Prevents Startup Items and Login Items from opening automatically

• Disables user-installed fonts 

• Deletes font caches, kernel cache, and other system cache files

NOTE 2: if you have a wireless keyboard with rechargeable batteries connect it with its charging cable before booting into Safe Mode. This makes it act as a wired keyboard as will insure a successful boot into Safe Mode.

zhané1234 wrote:

Hello.

I tried that but it changed back when I rebooted normally.

Some third party software modification that runs when booted normally (they don't run in Safe Mode) may be causing this problem.

Please run Etrecheck and post its full report here. Use the "additional text" button and paste the report into the text box.

To reitterate Luis's request download and run Etrecheck. 

Copy and paste the results into your reply. Etrecheck is a diagnostic tool that was developed by one of the most respected users here in the ASC and recommended by Apple Support  to provide a snapshot of the system and help identify the more obvious culprits that can adversely affect a Mac's performance.

Copy the report

and use the Additional Text button to paste the report in your reply.

Then we can examine the report and see if we can determine the cause of the problem.